DevTools: Added "async" flag for some methods handled in browser.

This flag will be used by code generator in content.
Removed unnecessary "optional" from Input.emulateTouchFromMouseEvent

BUG=405566

Review URL: https://codereview.chromium.org/565163002

git-svn-id: svn://svn.chromium.org/blink/trunk@182073 bbb929c8-8fbe-4397-9dbb-9b2b20218538