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 121 matching lines @@
   IPC_STRUCT_TRAITS_MEMBER(page_scale_factor)
   IPC_STRUCT_TRAITS_MEMBER(min_page_scale_factor)
   IPC_STRUCT_TRAITS_MEMBER(max_page_scale_factor)
   IPC_STRUCT_TRAITS_MEMBER(need_animate_scroll)
   IPC_STRUCT_TRAITS_MEMBER(need_invalidate_count)
   IPC_STRUCT_TRAITS_MEMBER(need_begin_frame)
   IPC_STRUCT_TRAITS_MEMBER(did_activate_pending_tree_count)
 IPC_STRUCT_TRAITS_END()
 
 // Messages sent from the browser to the renderer.
+// Synchronous IPCs are allowed here to the renderer compositor thread. See
+// design doc https://goo.gl/Tn81FW and crbug.com/526842 for details.
 
 IPC_SYNC_MESSAGE_ROUTED2_2(SyncCompositorMsg_HandleInputEvent,
                            content::SyncCompositorCommonBrowserParams,
                            IPC::WebInputEventPointer,
                            content::SyncCompositorCommonRendererParams,
                            content::InputEventAckState)
 
 IPC_SYNC_MESSAGE_ROUTED2_1(SyncCompositorMsg_BeginFrame,
                            content::SyncCompositorCommonBrowserParams,
                            cc::BeginFrameArgs,
@@ skipping 21 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)
 
+// The synchronous version is used to synchronize state from an earlier
+// asynchronous call only. This should be needed rarely so should prefer the
+// asynchronous version above in general.
+IPC_SYNC_MESSAGE_ROUTED1_1(SyncCompositorMsg_SynchronousUpdateState,
+                           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)