Introduce a field trial to strictly block "blockable" mixed content.
This patch adds a field trial group to control the Blink-side
"StrictlyBlockBlockableMixedContent" setting, which governs whether or
not Blink asks its embedder whether or not blockable mixed content
should load.
This has the effect of disabling the mixed content shield UI completely
for users who land in the field trial. The intent is to enable the trial
for Canary and Dev, and see how it goes.
blink-dev thread: https://groups.google.com/a/chromium.org/d/msg/blink-dev/1yWQaXmkXpA/BDV1-dC9su4J
BUG=513163
R=jochen@chromium.org,philipj@opera.com
Review URL: https://codereview.chromium.org/
1251243005
Cr-Commit-Position: refs/heads/master@{#341077}