Reset RenderThreadImpl::gpu_va_context_provider_ before shuttind down the main message loop.

Reset RenderThreadImpl::gpu_va_context_provider_ before shuttind down the main message loop.

According to the stack trace in bug 467372, ~RenderThreadImpl() destructs
RenderThreadImpl::gpu_va_context_provider_, which touches the main message loop
that has been already shut down and crashes. To avoid the crash, this CL
clears RenderThreadImpl::gpu_va_context_provider_ before RenderThreadImpl
shuts down the main message loop.

BUG=467372
Committed: https://crrev.com/b73174505bc418b2066037b54087a46c381436b0
Cr-Commit-Position: refs/heads/master@{#322188}

[haraken, 2015-03-25 15:10:17]

haraken@chromium.org changed reviewers:
+ jochen@chromium.org

[haraken, 2015-03-25 15:10:17]

PTAL

[jochen (gone - plz use gerrit), 2015-03-25 15:47:14]

lgtm

[haraken, 2015-03-25 16:35:04]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2015-03-25 16:35:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1036773002/20001

[commit-bot: I haz the power, 2015-03-25 17:48:56]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-25 17:49:43]

Patchset 2 (id:??) landed as
https://crrev.com/b73174505bc418b2066037b54087a46c381436b0
Cr-Commit-Position: refs/heads/master@{#322188}