Add comment explianing memory policy on Mac.

content/common/gpu/gpu_memory_manager.cc

Index: content/common/gpu/gpu_memory_manager.cc
diff --git a/content/common/gpu/gpu_memory_manager.cc b/content/common/gpu/gpu_memory_manager.cc
index 9a1402c789e192e40d29a3d9bbb8751dd4d0281f..c17bf658f57a753b833764dd573796329c33c37b 100644
--- a/content/common/gpu/gpu_memory_manager.cc
+++ b/content/common/gpu/gpu_memory_manager.cc
@@ -765,6 +765,9 @@ void GpuMemoryManager::AssignSurfacesAllocations() {
 
     allocation.renderer_allocation.bytes_limit_when_visible =
         client_state->bytes_allocation_when_visible_;
+    // Use a more conservative memory allocation policy on Mac because the
+    // platform is unstable when under memory pressure.
+    // http://crbug.com/141377
     allocation.renderer_allocation.priority_cutoff_when_visible =
 #if defined(OS_MACOSX)
         GpuMemoryAllocationForRenderer::kPriorityCutoffAllowNiceToHave;