mac: Switch the default compiler to clang for everyone.

mac: Switch the default compiler to clang for everyone.

Also commit http://codereview.chromium.org/7890052/ as part of this CL,
so that new pch files are created for clang automatically.

BUG=none
TEST=devs get happier, not too many bots break

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=101239

[Avi (use Gerrit), 2011-09-08 15:33:44]

When I use this with Xcode 3 I find that sometimes I make a change and it takes
two times initiating a build before my changes are 'seen' by the compiler.

[Nico, 2011-09-14 22:44:18]

Thanks, Avi! For posterity, that was http://crbug.com/96315

Mark: I think things are ready enough to land this. I will clobber main
waterfall and perf builders manually on friday for now and come up with a better
way for that next week.

[Mark Mentovai, 2011-09-15 02:54:58]

LGTM

http://codereview.chromium.org/7843002/diff/3001/build/common.gypi
File build/common.gypi (right):

http://codereview.chromium.org/7843002/diff/3001/build/common.gypi#newcode564
build/common.gypi:564: # Enable clang on mac by default!
LGTM!