Always disable GPU compositing on chromecast audio only

We can't use GPU compositing without a GPU process. This fixes a problem
in cast_shell_internal_browsertests that's occurring after 966e62f6
("[scheduler] Enable prioritize until begin main frame by default.").

Change-Id: I36eb9f615775e19f81fe24f8a532085fb9483ed9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303874
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789299}

chromecast/browser/cast_browser_main_parts.cc

@@ -311,10 +311,10 @@ const DefaultCommandLineSwitch kDefaultSwitches[] = {
 #if BUILDFLAG(IS_CAST_AUDIO_ONLY)
     {switches::kDisableGpu, ""},
     {switches::kDisableSoftwareRasterizer, ""},
+    {switches::kDisableGpuCompositing, ""},
 #if defined(OS_ANDROID)
     {switches::kDisableFrameRateLimit, ""},
     {switches::kDisableGLDrawingForTests, ""},
-    {switches::kDisableGpuCompositing, ""},
     {cc::switches::kDisableThreadedAnimation, ""},
 #endif  // defined(OS_ANDROID)
 #endif  // BUILDFLAG(IS_CAST_AUDIO_ONLY)