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

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

[horo, 2014-07-10 07:25:00]

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

[commit-bot: I haz the power, 2014-07-10 07:25:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/horo@chromium.org/378363006/1

[commit-bot: I haz the power, 2014-07-10 07:27:25]

Change committed as 282265

[Yuta Kitamura, 2014-07-10 07:30:23]

On 2014/07/10 07:27:25, I haz the power (commit-bot) wrote:
> Change committed as 282265

It was too late but to be clear: not lgtm.

[Yuta Kitamura, 2014-07-10 07:31:00]

I'm going to revert this revert.

[Yuta Kitamura, 2014-07-10 07:34:16]

A revert of this CL has been created in
https://codereview.chromium.org/382743002/ by yutak@chromium.org.

The reason for reverting is: 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"..