[Fuchsia] Specify GrContext type when enabling vulkan

Bug: 1016958
Change-Id: I7d918b17cd8b6d6b0b0d8a471087d44e5af11d65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872487Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708417}

fuchsia/engine/context_provider_impl.cc

@@ -253,6 +253,8 @@ void ContextProviderImpl::Create(
     launch_command.AppendSwitch(switches::kEnableOopRasterization);
     launch_command.AppendSwitchASCII(switches::kUseGL,
                                      gl::kGLImplementationStubName);
+    launch_command.AppendSwitchASCII(switches::kGrContextType,
+                                     switches::kGrContextTypeVulkan);
   }
 
   if (enable_widevine) {