Mixed Content: Extract frame processing from ::shouldBlockFetch.

Mixed Content: Extract frame processing from ::shouldBlockFetch.

In order to break MixedContentChecker's dependency on LocalFrame, we
need to replace the existing checking mechanisms with static calls. This
patch extracts some of the logic from ::shouldBlockFetch that we'll need
to reuse for blocking WebSockets and form actions in future CLs.

------------------------------------------------------------------------
This was originally landed in https://codereview.chromium.org/566533003,
but reverted due to hard-to-track-down ChromeOS issues. I'll re-land it
in parts to narrow down the bits that caused problems.
------------------------------------------------------------------------

BUG=411184
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183268

[Mike West, 2014-10-02 10:33:03]

mkwst@chromium.org changed reviewers:
+ jochen@chromium.org

[Mike West, 2014-10-02 10:33:04]

Hey Jochen, I want to get started relanding
https://codereview.chromium.org/566533003. I'll break it up into chunks to make
it easier to track down which piece caused problems for ChromeOS.

Here's the first step. WDYT?

[jochen (gone - plz use gerrit), 2014-10-02 10:59:50]

lgtm

[Mike West, 2014-10-02 11:01:25]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2014-10-02 11:01:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/623683002/1

[commit-bot: I haz the power, 2014-10-02 12:14:00]

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

[commit-bot: I haz the power, 2014-10-02 12:14:00]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/2...)

[Mike West, 2014-10-06 12:27:47]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2014-10-06 12:28:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/623683002/20001

[commit-bot: I haz the power, 2014-10-06 13:33:34]

Committed patchset #2 (id:20001) as 183268