Alter the Layout of WebTextView to match webkit.

Bug:3085564

In order to treat the selection handles properly, the text in
WebTextView needs to line up with the text in webkit.  Use the
line spacing style, or the paint if none provided, to determine
the spacing for WebTextView's Layout.  When no line spacing is
provided, use the floating point values of ascent and descent,
rather than the rounded result, to determine spacing.

Requires a change in external/webkit:
https://android-git.corp.google.com/g/#change,80641

Change-Id: I51081835e128a0e938028c7c7aeb5111aa5f072c