mac: Force InitializeVideoToolbox() in the GPU's PreSandboxStartup().

Under the V2 sandbox architecture, there is technically nothing that
runs "pre-sandbox startup". But we hypothesize that deferring the
initialization of VideoToolbox to the first use is leading to hangs,
which the watchdog turns into crashes. By forcing initialization at this
call site, the GPU watchdog thread is not running, which is what happens
under the V1 sandbox architecture.

Bug: 871280
Change-Id: I7c1173ecf7752307c19ac15f81e9b1c699d4523b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932166Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719164}

content/gpu/gpu_main.cc

@@ -92,6 +92,8 @@
 #if defined(OS_MACOSX)
 #include "base/message_loop/message_pump_mac.h"
 #include "components/metal_util/test_shader.h"
+#include "content/public/common/content_features.h"
+#include "media/gpu/mac/vt_video_decode_accelerator_mac.h"
 #include "sandbox/mac/seatbelt.h"
 #include "services/service_manager/sandbox/mac/sandbox_mac.h"
 #endif
@@ -157,6 +159,13 @@ class ContentSandboxHelper : public gpu::GpuSandboxHelper {
     media::MediaFoundationVideoEncodeAccelerator::PreSandboxInitialization();
 #endif
 
+#if defined(OS_MACOSX)
+    if (base::FeatureList::IsEnabled(features::kMacV2GPUSandbox)) {
+      TRACE_EVENT0("gpu", "Initialize VideoToolbox");
+      media::InitializeVideoToolbox();
+    }
+#endif
+
     // On Linux, reading system memory doesn't work through the GPU sandbox.
     // This value is cached, so access it here to populate the cache.
     base::SysInfo::AmountOfPhysicalMemory();