Allow to have a transparent UA dependent background.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index b7786ec809bfc5444b2074a6dd0e55fd19260d21..b0186795d914cee7b849230abebfe3ee8c087f25 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -1048,6 +1048,11 @@ const char kMemoryPressureThresholdsMb[] = "memory-pressure-thresholds-mb";
 const char kTraceExportEventsToETW[] = "trace-export-events-to-etw";
 #endif
 
+#if defined(USE_AURA)
+// Set transparent color for render widget host view's background.
+const char kTransparentBackgroundRWHV[] = "transparent-background-rwhv";
+#endif
+
 // Don't dump stuff here, follow the same order as the header.
 
 }  // namespace switches