Check if there is error on waiting for GL Fence.

ui/gl/gl_fence_arb.cc

Index: ui/gl/gl_fence_arb.cc
diff --git a/ui/gl/gl_fence_arb.cc b/ui/gl/gl_fence_arb.cc
index da13f107880d8ae0c996218af9d75fad46470932..c8a82989e2fc1a47b125881d5a937476903b3653 100644
--- a/ui/gl/gl_fence_arb.cc
+++ b/ui/gl/gl_fence_arb.cc
@@ -28,7 +28,9 @@ bool GLFenceARB::HasCompleted() {
   // We could potentially use glGetSynciv here, but it doesn't work
   // on OSX 10.7 (always says the fence is not signaled yet).
   // glClientWaitSync works better, so let's use that instead.
-  return glClientWaitSync(sync_, 0, 0) != GL_TIMEOUT_EXPIRED;
+  GLenum result = glClientWaitSync(sync_, GL_SYNC_FLUSH_COMMANDS_BIT, 0);

[no sievers, 2014/10/31 18:49:32]

GL_SYNC_FLUSH_COMMANDS_BIT doesn't work here because we might call this function
with a different context current than the one we created the fence in.
Also it's not needed since we call glFlush() in the constructor.

+  DCHECK_NE(GL_WAIT_FAILED, result);

[no sievers, 2014/10/31 18:49:32]

The spec says it can only fail if you use an illegal flag, which we don't, or if
|sync| is not the name of a sync object which we handle in line 24.

So not sure why it would fail. Maybe some driver internal bug? I wonder if it
would also generate a GL error in that case which we should then clear here.

+  return result != GL_TIMEOUT_EXPIRED;
 }
 
 void GLFenceARB::ClientWait() {