MacViews: Tweak kDragToEndIfOutsideVerticalBounds for single line fields.

Currently the direction to extend is based off the y-position. Dragging
above extends to the logical start and dragging below extends to the
logical end. This matches <textarea> and single-line <input> fields in
webcontents, but does not match single line Cocoa text fields such as
the current Chrome Mac omnibox.

Instead, compare x positions of the selection start and the mouse
cursor. For single line RenderText, extend to the end that is in the
visual direction towards the mouse cursor.

BUG=600166

Review-Url: https://codereview.chromium.org/2903193003
Cr-Commit-Position: refs/heads/master@{#475260}
3 files changed