Add support for page theme color to WebView and WebChromeClient.

Will only be implemented by system WebView, so users will have to call
the isThemeColorSupported() method in order to check whether the current
WebView implementation supports this.

Change-Id: Iaf4c3b8e4b8b7d8a74303b32d1c17c836dc744da
(cherry picked from commit 8a47a2fe)