Fix common.gypi when run with target_arch=mips

Fix common.gypi when run with target_arch=mips

The CXX variable is already set later on in this file
in make_global_settings.

The mips build was broken by a recent CL:
 https://codereview.chromium.org/1158283003

TEST=GYP_DEFINES=target_arch=mipsel gyp_chromium && ninja -C out/Release chrome
BUG=496273
Committed: https://crrev.com/15156343a810317288c08d176dd0cb26b75bb2bf
Cr-Commit-Position: refs/heads/master@{#332689}

[Sam Clegg, 2015-06-03 19:29:48]

sbc@chromium.org changed reviewers:
+ jonross@chromium.org

[Sam Clegg, 2015-06-03 19:45:32]

sbc@chromium.org changed reviewers:
+ dpranke@chromium.org

[Sam Clegg, 2015-06-03 19:45:32]

+dpranke for build/OWNERS

[jonross, 2015-06-03 19:49:48]

On 2015/06/03 19:45:32, Sam Clegg wrote:
> +dpranke for build/OWNERS

LGTM

[Dirk Pranke, 2015-06-03 20:05:02]

rubber-stamp lgtm.

[Sam Clegg, 2015-06-03 20:06:25]

The CQ bit was checked by sbc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-03 20:07:59]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1166523004/40001

[commit-bot: I haz the power, 2015-06-03 21:14:15]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-03 21:14:16]

Dry run: This issue passed the CQ dry run.

[Sam Clegg, 2015-06-03 21:21:30]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-06-03 21:23:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1166523004/40001

[commit-bot: I haz the power, 2015-06-03 21:29:12]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-03 21:30:06]

Patchset 3 (id:??) landed as
https://crrev.com/15156343a810317288c08d176dd0cb26b75bb2bf
Cr-Commit-Position: refs/heads/master@{#332689}