GitLab

1. Sticky keys
2. Tap dragging

Changes in this CL,

1. Add four functions in AccessibilityDelegate interface.
   This is used to show above two a11y features in system menu.

BUG=632107

Review-Url: https://codereview.chromium.org/2802783002
Cr-Commit-Position: refs/heads/master@{#462961}