Add a flag to allow renderer to use software compositor when GL compositor doesn't work.

content/shell/common/shell_switches.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/shell/common/shell_switches.h"
 
 namespace switches {
 
 // Allow access to external pages during layout tests.
 const char kAllowExternalPages[] = "allow-external-pages";
 
 // Check whether all system dependencies for running layout tests are met.
 const char kCheckLayoutTestSysDeps[] = "check-layout-test-sys-deps";
 
 // Tells Content Shell that it's running as a content_browsertest.
 const char kContentBrowserTest[] = "browser-test";
 
 // Makes Content Shell use the given path for its data directory.
 const char kContentShellDataPath[] = "data-path";
 
 // Request pages to be dumped as text once they finished loading.
 const char kDumpRenderTree[] = "dump-render-tree";
 
 // Enable accelerated 2D canvas.
 const char kEnableAccelerated2DCanvas[] = "enable-accelerated-2d-canvas";
 
-// Alias for kEnableSoftwareCompositingGLAdapter.
-const char kEnableSoftwareCompositing[] = "enable-software-compositing";
-
 // Encode binary layout test results (images, audio) using base64.
 const char kEncodeBinary[] = "encode-binary";
 
 // Exposes the window.internals object to JavaScript for interactive development
 // and debugging of layout tests that rely on it.
 const char kExposeInternalsForTesting[]     = "expose-internals-for-testing";
 
 // Save results when layout-as-browser tests fail.
 const char kOutputLayoutTestDifferences[] = "output-layout-test-differences";
 
 }  // namespace switches