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.

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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.
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BUG=411184

Review URL: https://codereview.chromium.org/623683002

git-svn-id: svn://svn.chromium.org/blink/trunk@183268 bbb929c8-8fbe-4397-9dbb-9b2b20218538