Mac: Fix accidental changes to fullscreen logic from refactor. (reland)

Mac: Fix accidental changes to fullscreen logic from refactor. (reland)

This CL was reverted due to test failures. Under inspection, the test proved to
already be flaky. The test has been disabled. See http://crbug.com/415422 for
more details.

--------------Original CL Description-----------------

The menu item "Enter Presentation Mode" has different effects in 10.6 and
10.7+. In 10.6, the menu item invokes Immersive Fullscreen. In 10.7+, the menu
item invokes AppKit Fullscreen. Similar logic applies to fullscreen invoked by
an extension.

I accidentally changed this logic during the fullscreen refactor by removing
the 10.6 vs 10.7+ conditional. This CL adds back the conditional. I also
updated the code to explicitly indicate the mechanism that is invoking
fullscreen to prevent similar future mistakes.

Original CL: https://codereview.chromium.org/575653003/

BUG=NONE
TBR=rsesek@chromium.org

Review URL: https://codereview.chromium.org/576393002

Cr-Commit-Position: refs/heads/master@{#295598}