Break out Chromoting plugin's cursor & mouse-lock handling.

Break out Chromoting plugin's cursor & mouse-lock handling.

This CL breaks cursor & mouse-lock handling out of PepperInputHandler:
- PepperCursorSetter is added to set the native PPAPI cursor.
  Callers may provide an alternate stub to delegate to if the PPAPI cursor
  cannot be set; ChromotingInstance uses this for oversized cursor rendering.
- EmptyCursorFilter is added to normalize transparent cursors to (0x0).
- PepperMouseLocker is added, to handle mouse-lock mode.
  This hooks into the cursor pipeline in order to detect (non)empty cursor
  shapes and engage mouse-lock accordingly, and to control whether any cursor
  is rendered locally.
- ChromotingInstance is updated to handle rceived cursors via these helpers,
  so that it need only directly handle delegated oversized cursors itself.

BUG=138108, 429322

Review URL: https://codereview.chromium.org/695583005

Cr-Commit-Position: refs/heads/master@{#305463}