Add a command line flag to enable WebGL overlay support.

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 b1223109de9d9f313ec993d464b3de33914dc1eb..3ff4073f6e286dc2e786ed336326abf986ed8d5b 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -574,6 +574,9 @@ const char kEnableWebAnimationsSVG[]        = "enable-web-animations-svg";
 // Enables WebGL extensions not yet approved by the community.
 const char kEnableWebGLDraftExtensions[] = "enable-webgl-draft-extensions";
 
+// Enables WebGL rendering into a scanout buffer for overlay support.
+const char kEnableWebGLImageChromium[] = "enable-webgl-image-chromium";
+
 // Enables Web MIDI API.
 const char kEnableWebMIDI[]                 = "enable-web-midi";