Enable new-style pixel layout tests always.

They are disabled on android forcing us to keep a whole heap of legacy
code around that is not even understandable at this point (the whole
CompositeAndReadbackAsync() path). In order to remove this old code,
which is also dead when Android layout tests aren't running, turn the
new mode on always. If pixel-based Android layout tests are wanted to
run, they should resolve whatever is busted for them in the new path
rather than keeping a whole other compositing mode around just for
them.

I will follow up with CLs to remove the flag once this proof of concept
lands, and to remove all the dead code in RenderWidget, WebTestSupport,
etc.

R=piman@chromium.org

Bug: 912193
Change-Id: I4bfb14101073ea5086ada901cf618ef1c5d08d2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542229Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645315}

content/shell/app/shell_main_delegate.cc

@@ -214,13 +214,7 @@ bool ShellMainDelegate::BasicStartupComplete(int* exit_code) {
     command_line.AppendSwitch(cc::switches::kEnableGpuBenchmarking);
     command_line.AppendSwitch(switches::kEnableLogging);
     command_line.AppendSwitch(switches::kAllowFileAccessFromFiles);
-#if !defined(OS_ANDROID)
-    // TODO(crbug/567947) Enable display compositor pixel dumps for Android
-    // once testing becomes possible on post-kitkat OSes, and once we've
-    // had a chance to debug the web test failures that occur when this
-    // flag is present.
     command_line.AppendSwitch(switches::kEnableDisplayCompositorPixelDump);
-#endif
     // only default to a software GL if the flag isn't already specified.
     if (!command_line.HasSwitch(switches::kUseGpuInTests) &&
         !command_line.HasSwitch(switches::kUseGL)) {