android webview: Keep zoom methods synchronous

content/common/android/sync_compositor_messages.h

Index: content/common/android/sync_compositor_messages.h
diff --git a/content/common/android/sync_compositor_messages.h b/content/common/android/sync_compositor_messages.h
index 2e9af1a9c768725949ee63e37c298c3291518f9c..36ca6b29acfdebf7faa36bca5d77b9bf57058a03 100644
--- a/content/common/android/sync_compositor_messages.h
+++ b/content/common/android/sync_compositor_messages.h
@@ -180,6 +180,10 @@ IPC_SYNC_MESSAGE_ROUTED2_3(SyncCompositorMsg_DemandDrawSw,
 IPC_MESSAGE_ROUTED1(SyncCompositorMsg_UpdateState,
                     content::SyncCompositorCommonBrowserParams)
 
+IPC_SYNC_MESSAGE_ROUTED1_1(SyncCompositorMsg_SynchronousUpdateState,

[dcheng, 2016/03/31 19:51:56]

I think we need a link to a design doc or overview somewhere. I recall being
convinced that sync IPC from the browser to the renderer for compositor was OK
but I don't recall why without doing a bunch of research again.

[dcheng, 2016/03/31 19:52:22]

Also, maybe some comments describing why we need both UpdateState and
SynchronousUpdateState would be good.

[boliu, 2016/03/31 20:16:34]

Good call on both counts.

Added link to bug and design doc above on Line 141. And added a comment here
that SynchronousUpdateState should be needed rarely needed, and should used the
async version in general.

+                           content::SyncCompositorCommonBrowserParams,
+                           content::SyncCompositorCommonRendererParams)
+
 // -----------------------------------------------------------------------------
 // Messages sent from the renderer to the browser.