Fix dual GPU switching behavior on Mac for in-process-gpu mode

BUG=759634
TEST=startup.warm.blank_page
R=kbr@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I050ced97dbba071b5fb32324239d45d43dd72f6d
Reviewed-on: https://chromium-review.googlesource.com/726499Reviewed-by: ccameron chromium <ccameron@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509903}

gpu/ipc/service/gpu_init.cc

@@ -283,6 +283,10 @@ void GpuInit::InitializeInProcess(base::CommandLine* command_line,
 #endif
     gpu_feature_info_ = gpu::ComputeGpuFeatureInfo(gpu_info_, command_line);
   }
+  if (gpu::SwitchableGPUsSupported(gpu_info_, *command_line)) {
+    gpu::InitializeSwitchableGPUs(
+        gpu_feature_info_.enabled_gpu_driver_bug_workarounds);
+  }
 
   if (!gl::init::InitializeGLNoExtensionsOneOff()) {
     VLOG(1) << "gl::init::InitializeGLNoExtensionsOneOff failed";