MIPS: Workaround for bug in MIPS Android compiler

This patch fixes an issue with compilation with MIPS Android compiler
NDK r12b, gcc version mipsel-linux-android-g++ (GCC) 4.9.x 20150123
(prerelease).
The problem has been reported to the compiler team, the workaround
should be reverted when the issue with compiler is resolved.

BUG=

Review-Url: https://codereview.chromium.org/2652673002
Cr-Commit-Position: refs/heads/master@{#42732}

src/heap/store-buffer.h

@@ -202,8 +202,8 @@ class StoreBuffer {
 
   // Callbacks are more efficient than reading out the gc state for every
   // store buffer operation.
-  std::function<void(StoreBuffer*, Address)> insertion_callback;
-  std::function<void(StoreBuffer*, Address, Address)> deletion_callback;
+  void (*insertion_callback)(StoreBuffer*, Address);
+  void (*deletion_callback)(StoreBuffer*, Address, Address);
 };
 
 }  // namespace internal