ahat: add support for diffing two heap dumps.

ahat now has the option to specify a --baseline hprof file to use as
the basis for comparing two heap dumps. When a baseline hprof file is
provided, ahat will highlight how the heap dump has changed relative
to the hprof file.

Differences that are highlighted include:
* overall heap sizes
* total bytes and number of allocations by type
* new and deleted instances of a given type
* retained sizes of objects
* instance fields, static fields, and array elements of modified objects

Also:
* Remove support for showing NativeAllocations, because I haven't ever
  found it to be useful, it is not obvious what a "native" allocation
  is, and I don't feel like adding diff support for them.
* Remove help page. Because it is outdated, not well maintained, and
  not very helpful in the first place.

Test: m ahat-test
Test: Run in diff mode for tests and added new tests for diff.
Test: Manually run with and without diff mode on heap dumps from system server.
Bug: 33770653
Change-Id: Id9a392ac75588200e716bbc3edbae6e9cd97c26b

1 files changed, 3 insertions, 1 deletions

diff --git a/tools/ahat/test/OverviewHandlerTest.java b/tools/ahat/test/OverviewHandlerTest.java
index a46bfce07d..c2f773b64b 100644
--- a/tools/ahat/test/OverviewHandlerTest.java
+++ b/tools/ahat/test/OverviewHandlerTest.java
@@ -26,7 +26,9 @@ public class OverviewHandlerTest {
   @Test
   public void noCrash() throws IOException {
     AhatSnapshot snapshot = TestDump.getTestDump().getAhatSnapshot();
-    AhatHandler handler = new OverviewHandler(snapshot, new File("my.hprof.file"));
+    AhatHandler handler = new OverviewHandler(snapshot,
+        new File("my.hprof.file"),
+        new File("my.base.hprof.file"));
     TestHandler.testNoCrash(handler, "http://localhost:7100");
   }
 }