Extract FrameSelection::ComputeAbsoluteBounds method

To be used in WebViewImpl and WebFrameWidgetImpl, which
currently implement it almost identically.

This will also allow to call ComputeAbsoluteBounds from
core/, reducing dependencies from core/ to core/exported.

Bug: 629721
Change-Id: I6ad23dba7af2126d85f8a300deefb467d1dcd185
Reviewed-on: https://chromium-review.googlesource.com/779863


Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518301}