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

Review URL: https://codereview.chromium.org/1036773002

Cr-Commit-Position: refs/heads/master@{#322188}