android webview: Keep zoom methods synchronous

content/common/android/sync_compositor_messages.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 
 #include "base/memory/shared_memory_handle.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/output/compositor_frame_ack.h"
@@ skipping 162 matching lines @@
 IPC_SYNC_MESSAGE_ROUTED2_3(SyncCompositorMsg_DemandDrawSw,
                            content::SyncCompositorCommonBrowserParams,
                            content::SyncCompositorDemandDrawSwParams,
                            bool /* result */,
                            content::SyncCompositorCommonRendererParams,
                            cc::CompositorFrame)
 
 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.
 
 IPC_MESSAGE_ROUTED1(SyncCompositorHostMsg_UpdateState,
                     content::SyncCompositorCommonRendererParams)
 
 IPC_MESSAGE_ROUTED2(SyncCompositorHostMsg_OverScroll,
                     content::SyncCompositorCommonRendererParams,
                     content::DidOverscrollParams)