Move ArtField to native

Add linear alloc. Moved ArtField to be native object. Changed image
writer to put ArtFields after the mirror section.

Savings:
2MB on low ram devices
4MB on normal devices

Total PSS measurements before (normal N5, 95s after shell start):
Image size: 7729152 bytes
23112 kB: .NonMoving
23212 kB: .NonMoving
22868 kB: .NonMoving
23072 kB: .NonMoving
22836 kB: .NonMoving
19618 kB: .Zygote
19850 kB: .Zygote
19623 kB: .Zygote
19924 kB: .Zygote
19612 kB: .Zygote
Avg: 42745.4 kB

After:
Image size: 7462912 bytes
17440 kB: .NonMoving
16776 kB: .NonMoving
16804 kB: .NonMoving
17812 kB: .NonMoving
16820 kB: .NonMoving
18788 kB: .Zygote
18856 kB: .Zygote
19064 kB: .Zygote
18841 kB: .Zygote
18629 kB: .Zygote
3499 kB: .LinearAlloc
3408 kB: .LinearAlloc
3424 kB: .LinearAlloc
3600 kB: .LinearAlloc
3436 kB: .LinearAlloc
Avg: 39439.4 kB

No reflection performance changes.

Bug: 19264997
Bug: 17643507

Change-Id: I10c73a37913332080aeb978c7c94713bdfe4fe1c
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index b9153c1..83da5a8 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -23,6 +23,7 @@
 #include <memory>
 #include <vector>
 
+#include "art_field-inl.h"
 #include "base/allocator.h"
 #include "base/dumpable.h"
 #include "base/histogram-inl.h"
@@ -58,7 +59,6 @@
 #include "heap-inl.h"
 #include "image.h"
 #include "intern_table.h"
-#include "mirror/art_field-inl.h"
 #include "mirror/class-inl.h"
 #include "mirror/object.h"
 #include "mirror/object-inl.h"
@@ -233,7 +233,7 @@
       CHECK_GT(oat_file_end_addr, image_space->End());
       requested_alloc_space_begin = AlignUp(oat_file_end_addr, kPageSize);
     } else {
-      LOG(WARNING) << "Could not create image space with image file '" << image_file_name << "'. "
+      LOG(ERROR) << "Could not create image space with image file '" << image_file_name << "'. "
                    << "Attempting to fall back to imageless running. Error was: " << error_msg;
     }
   }
@@ -482,7 +482,7 @@
                                       non_moving_space_->GetMemMap());
     if (!no_gap) {
       MemMap::DumpMaps(LOG(ERROR));
-      LOG(FATAL) << "There's a gap between the image space and the main space";
+      LOG(FATAL) << "There's a gap between the image space and the non-moving space";
     }
   }
   if (running_on_valgrind_) {
@@ -2708,12 +2708,12 @@
           // Print which field of the object is dead.
           if (!obj->IsObjectArray()) {
             mirror::Class* klass = is_static ? obj->AsClass() : obj->GetClass();
-            CHECK(klass != NULL);
-            mirror::ObjectArray<mirror::ArtField>* fields = is_static ? klass->GetSFields()
-                                                                      : klass->GetIFields();
-            CHECK(fields != NULL);
-            for (int32_t i = 0; i < fields->GetLength(); ++i) {
-              mirror::ArtField* cur = fields->Get(i);
+            CHECK(klass != nullptr);
+            auto* fields = is_static ? klass->GetSFields() : klass->GetIFields();
+            auto num_fields = is_static ? klass->NumStaticFields() : klass->NumInstanceFields();
+            CHECK_EQ(fields == nullptr, num_fields == 0u);
+            for (size_t i = 0; i < num_fields; ++i) {
+              ArtField* cur = &fields[i];
               if (cur->GetOffset().Int32Value() == offset.Int32Value()) {
                 LOG(ERROR) << (is_static ? "Static " : "") << "field in the live stack is "
                           << PrettyField(cur);