Use DLOG to report lost shared UI context

The GpuProcessTransportFactory::OnLostMainThreadSharedContext gets
called in a valid codepathes inside compositor - this message
should not be reported to regular users.

This causes a lot of confusion to Chrome Puppeteer clients who
attribute their issues to the first error they see in terminal:
https://github.com/GoogleChrome/puppeteer/issues/1925

Change-Id: Ib6c6862e6fb6a71fae388675d6dc709cf99503c8
Reviewed-on: https://chromium-review.googlesource.com/c/1333832
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607723}

content/browser/compositor/gpu_process_transport_factory.cc

@@ -960,8 +960,6 @@ GpuProcessTransportFactory::CreatePerCompositorData(
 }
 
 void GpuProcessTransportFactory::OnLostMainThreadSharedContext() {
-  LOG(ERROR) << "Lost UI shared context.";
-
   // Keep old resources around while we call the observers, but ensure that
   // new resources are created if needed.
   // Kill shared contexts for both threads in tandem so they are always in
@@ -1004,6 +1002,8 @@ GpuProcessTransportFactory::SharedVulkanContextProvider() {
 #endif
 
 void GpuProcessTransportFactory::OnContextLost() {
+  DLOG(ERROR) << "Lost UI shared context.";
+
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::BindOnce(&GpuProcessTransportFactory::OnLostMainThreadSharedContext,