Replaced ownership of PinchViewport from WebView to FrameHost.

Removed PinchViewport's dependency on WebViewImpl and replaced it with
FrameHost, foreshadowing its move to FrameHost.

BUG=349941

Review URL: https://codereview.chromium.org/186123005

git-svn-id: svn://svn.chromium.org/blink/trunk@168693 bbb929c8-8fbe-4397-9dbb-9b2b20218538