Add Pepper Flash plugin support (issue #1586).

A system-wide installation of the Pepper Flash plugin is available from Adobe
for Windows and Mac OS X platforms as a separate download. To enable automatic
detection and loading of the system-wide installation pass the
`--enable-system-flash` command-line flag.

The Pepper Flash plugin can also be loaded by specifying the file path and
version via the `--ppapi-flash-path=<path> --ppapi-flash-version=<version>`
command-line flags. The version can be identified by viewing the
manifest.json file in the same directory as the Pepper Flash plugin library.

libcef/browser/browser_host_impl.h

@@ -415,6 +415,9 @@ class CefBrowserHostImpl : public CefBrowserHost,
       content::WebContents* web_contents,
       const content::MediaStreamRequest& request,
       const content::MediaResponseCallback& callback) override;
+  bool CheckMediaAccessPermission(content::WebContents* web_contents,
+                                  const GURL& security_origin,
+                                  content::MediaStreamType type) override;
 
   // content::WebContentsObserver methods.
   using content::WebContentsObserver::BeforeUnloadFired;