Mac: Fix accidental changes to fullscreen logic from refactor.
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.
BUG=NONE
Review URL: https://codereview.chromium.org/
575653003
Cr-Commit-Position: refs/heads/master@{#295321}