Webview listen to Scroll event to update the viewport

In the case of unchanged layout, like replying in email/gmail,
scrolling should cause a viewport calculation update.
Otherwise, the content will be skewed.

Also, rename the mListener to mGlobalLayoutListener to avoid
confusion with added listener.

bug : 3143829

Change-Id: I7f5ee2e2cc9524e21cfffeef40b8650b6ee9a387