views::Label: Invalidate cached draw flags when background color is changed

views::Label: Invalidate cached draw flags when background color is changed

Transparency of the background color affects the result of |Label::ComputeDrawStringFlags()|.

BUG=447932
R=sky
NOTRY=true

Committed: https://crrev.com/8b51af8017e02cb46a17bff0980bebf973a23790
Cr-Commit-Position: refs/heads/master@{#311088}

[sky, 2015-01-12 17:00:55]

LGTM

[ckocagil, 2015-01-12 17:17:37]

The CQ bit was checked by ckocagil@chromium.org

[commit-bot: I haz the power, 2015-01-12 17:18:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/846853003/1

[commit-bot: I haz the power, 2015-01-12 17:23:30]

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

[commit-bot: I haz the power, 2015-01-12 17:23:31]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[ckocagil, 2015-01-12 19:46:16]

The CQ bit was checked by ckocagil@chromium.org

[commit-bot: I haz the power, 2015-01-12 19:47:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/846853003/1

[commit-bot: I haz the power, 2015-01-12 19:49:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-12 19:50:29]

Patchset 1 (id:??) landed as
https://crrev.com/8b51af8017e02cb46a17bff0980bebf973a23790
Cr-Commit-Position: refs/heads/master@{#311088}