. Use SANDBOXED as the default argument to GpuProcessHost::Get(), since
  that's the most common use.
. Remove unused ash::GPUSupport::GetGpuProcessHandles().
. Remove unused content::GpuDataManager::GetGpuProcessHandles().
. Replace GpuProcessHost::GetProcessHandles() with GetHasGpuProcess(),
  since the only remaining user (RenderMessageFilter) cares only about
  the existence of a gpu process, and not about what those processes are.

BUG=417532, 709332

Review-Url: https://codereview.chromium.org/2810593003
Cr-Commit-Position: refs/heads/master@{#463324}