Lacros: Enable WebGL rendering to scanout buffer

--enable-webgl-image-chromium flag enables WebGL content to be
eligible for overlay. WaylandOverlayDelegation feature (default on in
LaCrOS) needs it to delegate WebGL overlays to Ash.

If WaylandOverlayDelegation is off, or LaCrOS version is older, overlay
strategy will not pick WebGL quads as candidate and they should be
properly composited.

Change-Id: I59efdda46ba2254d258f29f0fb576fd4a3adaee7
Bug: 1063865
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510929Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822796}

chrome/browser/chromeos/crosapi/browser_manager.cc

@@ -318,6 +318,7 @@ void BrowserManager::StartWithLogFile(base::ScopedFD logfd) {
                                    "--lang=en-US",
                                    "--enable-crashpad",
                                    "--enable-wayland-ime",
+                                   "--enable-webgl-image-chromium",
                                    "--breakpad-dump-location=" + crash_dir};
 
   // CrAS is the default audio server in Chrome OS.