Minor visual improvements to text selection UI

Getting rid of the blue cursor lines drawn at the edges of text selection
highlight (when selecting) and on top of the blinking black cursor (when
editing).
Shifting the selection handles 2pts below the text baseline.
Offsetting the touch selection context menu by 2pts from the text highlight.

BUG=436047

Review URL: https://codereview.chromium.org/769393002

Cr-Commit-Position: refs/heads/master@{#307784}
2 files changed