ozone: Rename CreateStartupEventConverters() to StartProcessingEvents()

This function has a broader role than its name and comment suggest. No
events should be processed without a call to this function.

All chrome processes have an EventFactoryOzone; this provides the
platform-specific code to ui/events. The code should only actually dispatch
events in processes that call StartProcessingEvents().

BUG=none
TEST=ui_unittests

Review URL: https://codereview.chromium.org/50123007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231977