Prepare SyntheticPointerAction to handle mouse actions

This patch is to make SyntheticPointerAction handle mouse actions.

Next patch: Modify SyntheticGestureController to queue all the actions in one
SyntheticPointerAction until we see a PROCESS action, then pushing in a queue
and flush.

BUG=525187

Review-Url: https://codereview.chromium.org/2178153002
Cr-Commit-Position: refs/heads/master@{#410124}
4 files changed