Use stub GL draw/clear calls for browser tests that do not need pixels.

content/public/test/browser_test_base.cc

Index: content/public/test/browser_test_base.cc
diff --git a/content/public/test/browser_test_base.cc b/content/public/test/browser_test_base.cc
index a8733e4435b68f7d03b6fa915e795b7b647510df..60596a9df03fcc2de2095fcdc2bd68fac0e7662e 100644
--- a/content/public/test/browser_test_base.cc
+++ b/content/public/test/browser_test_base.cc
@@ -177,8 +177,7 @@ void BrowserTestBase::SetUp() {
   // Use test contexts for browser tests unless they override and force us to
   // use a real context.
   if (allow_test_contexts_ && !use_software_compositing_) {
-    content::ImageTransportFactory::InitializeForUnitTests(
-        scoped_ptr<ui::ContextFactory>(new ui::TestContextFactory));
+    command_line->AppendSwitch(switches::kDisableGLDrawingForTests);
   }
 #endif