Added pinch viewport offset to history item.

Added field to HistoryItem to save pinch viewport's offset within the main
frame and patched up the history system to be aware of the pinch viewport.

Also, removed "savedScrollAndScale" dead code from WebView

Follow-up Chromium patch: https://codereview.chromium.org/266673014

BUG=364112

Review URL: https://codereview.chromium.org/263853008

git-svn-id: svn://svn.chromium.org/blink/trunk@173757 bbb929c8-8fbe-4397-9dbb-9b2b20218538