cc: Fix race in SurfaceLayer unittests.

cc/layers/surface_layer_unittest.cc

Index: cc/layers/surface_layer_unittest.cc
diff --git a/cc/layers/surface_layer_unittest.cc b/cc/layers/surface_layer_unittest.cc
index f6e1eb6c32fb6b70606d87b9032e9106dc12d942..c3eae0b432eff914853a86cc8aa78485ca1c2b57 100644
--- a/cc/layers/surface_layer_unittest.cc
+++ b/cc/layers/surface_layer_unittest.cc
@@ -179,6 +179,7 @@ class SurfaceLayerSwapPromise : public LayerTreeTest {
   }
 
   void ChangeTree() {
+    base::AutoLock lock(lock_);

[danakj, 2015/01/06 15:57:40]

This still means ChangeTree and SwapBuffersOnThread may run in either order
which is not very desirable for a test.

Probably DidCommit should be DidCommitAndDrawFrame() so that SwapBuffersOnThread
always comes first? Or post the ChangeTree call from SwapBuffersOnThread
instead?

[sohanjg, 2015/01/08 12:05:41]

Yes, i have changed DidCommit to DidCommitAndDrawFrame.
I tried posting ChangeTree from SwapBuffersOnThread, but that hits an assert as
we do SetRootLayer in ChangeTree, and that will do a setneedscommit, which is
expected from Main thread.

     ++commit_count_;
     switch (commit_count_) {
       case 1:
@@ -204,6 +205,7 @@ class SurfaceLayerSwapPromise : public LayerTreeTest {
     if (satisfied.size() == 1) {
       // Eventually the one SurfaceSequence should be satisfied, but only
       // after the layer was removed from the tree, and only once.
+      base::AutoLock lock(lock_);
       EXPECT_EQ(1u, satisfied[0]);
       EXPECT_LE(1, commit_count_);
       EXPECT_FALSE(sequence_was_satisfied_);
@@ -229,6 +231,7 @@ class SurfaceLayerSwapPromise : public LayerTreeTest {
 
   SurfaceId required_id_;
   std::set<SurfaceSequence> required_set_;
+  base::Lock lock_;
 };
 
 // TODO(jbauman): Reenable on single thread once http://crbug.com/421923 is