Add nullptr check for GpuProcessHost when allocating GpuMemoryBuffer

This can return nullptr if the gpu process has failed to launch, or if
the browser main message loop has exited.

The other calls to GpuProcessHost::GetFromId do not dereference their
result without first checking against nullptr.

TBR=zmo (revied at https://codereview.chromium.org/2963433002/)

Bug: 723698
Change-Id: I556b8c937d9aaea8806902003209543b85823fb7
Reviewed-on: https://chromium-review.googlesource.com/595037Reviewed-by: ccameron chromium <ccameron@chromium.org>
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491529}

content/browser/gpu/browser_gpu_memory_buffer_manager.cc

@@ -356,6 +356,11 @@ void BrowserGpuMemoryBufferManager::CreateGpuMemoryBufferOnIO(
   }
 
   GpuProcessHost* host = GpuProcessHost::Get();
+  if (!host) {
+    DLOG(ERROR) << "Cannot allocate GpuMemoryBuffer with no GpuProcessHost.";
+    callback.Run(gfx::GpuMemoryBufferHandle());
+    return;
+  }
   // Note: Unretained is safe as IO thread is stopped before manager is
   // destroyed.
   host->CreateGpuMemoryBuffer(