Use cached memory for gcstress unwinds.

Chris added faster memory recently which does page-based caching.

Change-Id: I94f3baf48bef107a1994d03de47909e9a5433e83
diff --git a/runtime/backtrace_helper.cc b/runtime/backtrace_helper.cc
index 843b403..5b316fa 100644
--- a/runtime/backtrace_helper.cc
+++ b/runtime/backtrace_helper.cc
@@ -53,7 +53,7 @@
 
   explicit UnwindHelper(size_t max_depth)
       : arch_(unwindstack::Regs::CurrentArch()),
-        memory_(unwindstack::Memory::CreateProcessMemory(getpid())),
+        memory_(unwindstack::Memory::CreateProcessMemoryThreadCached(getpid())),
         jit_(unwindstack::CreateJitDebug(arch_, memory_)),
         dex_(unwindstack::CreateDexFiles(arch_, memory_)),
         unwinder_(max_depth, &maps_, memory_) {