use a gcc version check in 4.9 warning workaround

use a gcc version check in 4.9 warning workaround

Only disable the spurious warning on versions of GCC that
need it, so we don't hide real warnings or break older
compilers unnecessarily.

Followup from https://codereview.chromium.org/384843002/

BUG=392872
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=286347

[Mostyn Bramley-Moore, 2014-07-23 21:23:03]

@yuanlin, abarth: PTAL at this small improvement.

[yunlian, 2014-07-23 21:31:45]

lgtm

[Mostyn Bramley-Moore, 2014-07-28 07:29:27]

abarth: *ping*

[abarth-chromium, 2014-07-29 16:41:34]

lgtm

[commit-bot: I haz the power, 2014-07-29 16:43:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mostynb@opera.com/413023002/20001

[Mostyn Bramley-Moore, 2014-07-29 21:39:34]

The CQ bit was unchecked by mostynb@opera.com

[Mostyn Bramley-Moore, 2014-07-29 21:39:39]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2014-07-29 21:39:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mostynb@opera.com/413023002/20001

[commit-bot: I haz the power, 2014-07-29 23:56:08]

Change committed as 286347