Simplify overridable logic in SSLBlockingPage

Simplify overridable logic in SSLBlockingPage

Right now, SSLBlockingPage has both overridable_ and strict_enforcement_. 
This CL collapses both into overridable_, so that we only need to check one. 
However, strict_enforcement_ also remains on its own because it is checked 
separately in one place.

BUG=
R=palmer@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288456

[felt, 2014-08-07 14:41:03]

palmer: pls review.

[palmer, 2014-08-07 19:04:55]

lgtm

[felt, 2014-08-07 19:06:19]

The CQ bit was checked by felt@chromium.org

[commit-bot: I haz the power, 2014-08-07 19:11:59]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/felt@chromium.org/448163002/1

[commit-bot: I haz the power, 2014-08-07 22:42:43]

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

[commit-bot: I haz the power, 2014-08-07 23:44:15]

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

[commit-bot: I haz the power, 2014-08-07 23:44:27]

Try jobs failed on following builders:
  android_dbg_triggered_tests on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tri...)

[felt, 2014-08-08 22:45:15]

The CQ bit was checked by felt@chromium.org

[commit-bot: I haz the power, 2014-08-08 22:46:52]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/felt@chromium.org/448163002/1

[commit-bot: I haz the power, 2014-08-08 22:52:33]

Change committed as 288456