Force on partial raster for layout tests.

content/shell/app/shell_main_delegate.cc

Index: content/shell/app/shell_main_delegate.cc
diff --git a/content/shell/app/shell_main_delegate.cc b/content/shell/app/shell_main_delegate.cc
index 0401ad7f3634d3031fada347d827db28dba7b7f4..56738e195062df479efcc532011cfc8977b3eb8d 100644
--- a/content/shell/app/shell_main_delegate.cc
+++ b/content/shell/app/shell_main_delegate.cc
@@ -202,6 +202,8 @@ bool ShellMainDelegate::BasicStartupComplete(int* exit_code) {
     command_line.AppendSwitchASCII(switches::kHostResolverRules,
                                    "MAP *.test 127.0.0.1");
 
+    command_line.AppendSwitch(switches::kEnablePartialRaster);
+
     // Unless/until WebM files are added to the media layout tests, we need to
     // avoid removing MP4/H264/AAC so that layout tests can run on Android.
 #if !defined(OS_ANDROID)