Add command line option to enable PPAPI win32k lockdown.
This patch adds a command line option (enable-win32k-lockdown-mimetypes)
to selectively enable win32k lockdown on specific types of PPAPI content.
This will not currently allow any PPAPI process to work under lockdown
as further changes are necessary to remove usage of GDI.

BUG=523278

Review URL: https://codereview.chromium.org/1306243012

Cr-Commit-Position: refs/heads/master@{#349568}
7 files changed