Disable Clang's warnings about undefined pointer comparisons in Debug builds

Disable Clang's warnings about undefined pointer comparisons in Debug builds

The address of a reference, or the 'this' pointer, are always non-NULL
in well-defined C/C++ programs, and Clang warns about comparing them to NULL,
because the comparisons may be optimized away.

However, it can still be useful to do such comparisons in Debug builds.
For example, Skia does a lot of SkASSERT(&a), where a is a reference
parameter.

BUG=404271
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290361

[hans, 2014-08-18 19:24:35]

I first tried adding it to clang_warning_flags here, but that didn't work :/

[Nico, 2014-08-18 19:26:22]

lgtm

[hans, 2014-08-18 19:29:39]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2014-08-18 19:30:24]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hans@chromium.org/479293002/1

[commit-bot: I haz the power, 2014-08-18 20:45:12]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_gn_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[commit-bot: I haz the power, 2014-08-18 21:01:16]

Committed patchset #1 (1) as 290361