Ken points out that the GPU code does sync IPCs from the browser.

We chatted with Yuzhu and came up with:
1) disable it process wide
2) have something like base::ScopedAllowWait to enable it. Like the base class, the new one should be in a private section of a class and we should use a small list of OWNERS files to ensure that we have careful review of any sync IPCs from the browser.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/519639829bd17eaae4b147f7a96cbd1406fc6244

commit 519639829bd17eaae4b147f7a96cbd1406fc6244
Author: yzshen <yzshen@chromium.org>
Date: Tue Jun 21 19:37:10 2016

Mojo: add support for disallowing sync calls for a process.

This CLs introduce a new system API to expose the configuration whether sync calls are allowed to mojo apps.
The bindings check this configuration before processing a sync call (if DCHECK is enabled).

This CL also disables sync calls for the browser process.

BUG= 620733 

Review-Url: https://codereview.chromium.org/2084993002
Cr-Commit-Position: refs/heads/master@{#401076}

[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/base/threading/thread_restrictions.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/components/mus/common/gpu_service.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/content/browser/browser_main_loop.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/edk/embedder/embedder.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/edk/embedder/embedder.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/edk/embedder/entrypoints.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/edk/system/core.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/edk/system/core.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/mojo_public.gypi
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/c/system/functions.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/c/system/thunks.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/c/system/thunks.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/c/system/types.h
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/cpp/bindings/BUILD.gn
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/cpp/bindings/lib/interface_endpoint_client.cc
[modify] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/cpp/bindings/lib/router.cc
[add] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/cpp/bindings/lib/sync_call_restrictions.cc
[add] https://crrev.com/519639829bd17eaae4b147f7a96cbd1406fc6244/mojo/public/cpp/bindings/sync_call_restrictions.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1ebd04a612726d97423917c58ec209c64e0e5304

commit 1ebd04a612726d97423917c58ec209c64e0e5304
Author: yzshen <yzshen@chromium.org>
Date: Wed Jun 22 23:12:21 2016

Change ClipboardMus to use mojo::SyncCallRestrictions::ScopedAllowSyncCall.

BUG= 620733 

Review-Url: https://codereview.chromium.org/2085353003
Cr-Commit-Position: refs/heads/master@{#401449}

[modify] https://crrev.com/1ebd04a612726d97423917c58ec209c64e0e5304/base/threading/thread_restrictions.h
[modify] https://crrev.com/1ebd04a612726d97423917c58ec209c64e0e5304/mojo/public/cpp/bindings/sync_call_restrictions.h
[modify] https://crrev.com/1ebd04a612726d97423917c58ec209c64e0e5304/ui/views/mus/clipboard_mus.cc