Revert "MouseEvent Cleanup: Replace isSimulated with new SyntheticEventType value."

This reverts commit f4f316b59d15194c8caf75b0037276380e978f5a.

Broke webkit_tests fast/forms/input-image-submit.html
on WebKit XP and WebKit Linux 32

BUG=405990
TBR=tkent, rbyers@chromium.org, igsolla@chromium.org, eseidel

Review URL: https://codereview.chromium.org/543793004

git-svn-id: svn://svn.chromium.org/blink/trunk@181396 bbb929c8-8fbe-4397-9dbb-9b2b20218538