Make scrolling textfields work better in the browser.

The touch slop seems to be too large for scrolling
a small textfield, so use a smaller number.  Also,
in WebView, create viewToContentDimension, and use
it for determining the scroll position of the text
field.

Partial fix for http://b/issue?id=2133049

Change-Id: I0ded3be264b179bad39301e6adce86851b649a42