Update touch selection to only modify one selection point at a time.

This is made in preparation for enabling selection auto scroll using touch
handles.

BUG=340658

Review URL: https://codereview.chromium.org/653383002

git-svn-id: svn://svn.chromium.org/blink/trunk@184179 bbb929c8-8fbe-4397-9dbb-9b2b20218538