Introduce a field trial to strictly block "blockable" mixed content.

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

Committed: https://crrev.com/2384c822ac23e8bd427c65ff5e7edd28d07f8b61
Cr-Commit-Position: refs/heads/master@{#341077}

[Mike West, 2015-07-23 13:42:21]

Philip: This is what the //content side of the patch looks like. I'm sure it'll
break some tests, and I want to wait to see how people react on the thread, but
if we make this change, it'll probably look something like this. :)

[philipj_slow, 2015-07-23 15:04:06]

I see, non-owner LGTM.

[Mike West, 2015-07-24 07:05:28]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-24 07:05:47]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251243005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251243005/1

[commit-bot: I haz the power, 2015-07-24 07:57:25]

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

[commit-bot: I haz the power, 2015-07-24 07:57:26]

Dry run: Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Mike West, 2015-07-24 08:35:52]

On 2015/07/24 at 07:57:26, commit-bot wrote:
> Dry run: Try jobs failed on following builders:
>   linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

Based on some feedback, this patch probably won't be what we do. But I'll leave
it here until we figure out what the right way to approach this is in the
embedder.

[Mike West, 2015-07-25 04:54:22]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-25 04:54:25]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251243005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251243005/20001

[commit-bot: I haz the power, 2015-07-25 06:00:03]

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

[commit-bot: I haz the power, 2015-07-25 06:00:03]

Dry run: This issue passed the CQ dry run.

[Mike West, 2015-07-29 14:45:12]

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

[Mike West, 2015-07-29 14:45:12]

Hey Jochen! felt@ suggested locking this change behind finch, and rolling it out
to Canary and Dev only, to see who screams.

WDYT about this implementation of that suggestion?

[Mike West, 2015-07-29 14:45:29]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-29 14:46:03]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251243005/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251243005/40001

[jochen (gone - plz use gerrit), 2015-07-29 14:54:29]

lgtm

[felt, 2015-07-29 15:08:14]

lgtm

[Mike West, 2015-07-29 15:26:31]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-29 15:26:53]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251243005/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251243005/60001

[commit-bot: I haz the power, 2015-07-29 16:41:19]

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

[commit-bot: I haz the power, 2015-07-29 16:41:20]

Dry run: This issue passed the CQ dry run.

[Mike West, 2015-07-30 05:30:42]

The CQ bit was checked by mkwst@chromium.org

[Mike West, 2015-07-30 05:30:42]

The patchset sent to the CQ was uploaded after l-g-t-m from philipj@opera.com,
jochen@chromium.org, felt@chromium.org
Link to the patchset: https://codereview.chromium.org/1251243005/#ps60001
(title: "Test.")

[commit-bot: I haz the power, 2015-07-30 05:31:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251243005/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251243005/60001

[commit-bot: I haz the power, 2015-07-30 07:26:52]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-07-30 07:28:06]

Patchset 4 (id:??) landed as
https://crrev.com/2384c822ac23e8bd427c65ff5e7edd28d07f8b61
Cr-Commit-Position: refs/heads/master@{#341077}