Revert of Revert of Turn on clang by default on linux, 3rd try.

Revert of Revert of Turn on clang by default on linux, 3rd try. (https://codereview.chromium.org/378363006/)

Reason for revert:
This change should not have been reverted, since
it was easy to just disable these tests. thakis
explicitly requested "please ping thakis before
reverting it".

Original issue's description:
> Revert of Turn on clang by default on linux, 3rd try. (https://codereview.chromium.org/379413002/)
> 
> Reason for revert:
> NaClBrowserTestNonSfiMode.Messaging and NaClBrowserTestNonSfiMode.Irt failed.
> 
> http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/12124/steps/browser_tests/logs/Irt
> http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/12124/steps/browser_tests/logs/Messaging
> 
> Original issue's description:
> > Turn on clang by default on linux, 3rd try.
> > 
> > This is mostly to collect performance and size data for now, and to find out
> > if more bots need work. Unless things look really good, I'm going to revert
> > this by thursday morning (PDT).
> > 
> > BUG=360311
> > TBR=ajwong@chromium.org
> > 
> > Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282246
> 
> TBR=awong@chromium.org,thakis@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=360311
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282265

TBR=awong@chromium.org,thakis@chromium.org,horo@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=360311
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282266

[Yuta Kitamura, 2014-07-10 07:34:17]

Created Revert of Revert of Turn on clang by default on linux, 3rd try.

[commit-bot: I haz the power, 2014-07-10 07:34:49]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yutak@chromium.org/382743002/1

[commit-bot: I haz the power, 2014-07-10 07:36:44]

Change committed as 282266