A bunch of remaining parts of extension content verification (Reland)

A bunch of remaining parts of extension content verification (Reland)

-The real guts of content_hash_fetcher.cc, which fetches the
 verified_contents.json file from the webstore if needed and also runs
 tasks to compute and cache the block-level hashes of all files in an
 extension.

-The real guts of content_hash_reader.cc, which uses the work done by
 the content_hash_fetcher during validation of extension file content as
 it's read off of disk at time of use.

-Code to avoid verifying transcoded files (images used in browser
 process, and message catalogs).

-Don't allow downgrade of mode via kForceFieldTrials command line switch

-Various bits of plumbing to support all of the above


This is a re-land with fixes; original review was:
  https://codereview.chromium.org/289533003


BUG=369895, 373854
R=rockot@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270937

[asargent_no_longer_on_chrome, 2014-05-15 21:55:23]

Ken, can you take a look at the small diffs here from the first CL? I uploaded
the original in patch set 1, so you can just look at the diffs between patch
sets 1 and 2 (only 2 files). 

Also I have sentenced both of us to a punishment of writing on the whiteboard
100 times: "Do not do actual work inside DCHECKS. Do not do actual work inside
DCHECKS. ..."

[Ken Rockot(use gerrit already), 2014-05-15 21:58:57]

On 2014/05/15 21:55:23, Antony Sargent wrote:
> Ken, can you take a look at the small diffs here from the first CL? I uploaded
> the original in patch set 1, so you can just look at the diffs between patch
> sets 1 and 2 (only 2 files). 
> 
> Also I have sentenced both of us to a punishment of writing on the whiteboard
> 100 times: "Do not do actual work inside DCHECKS. Do not do actual work inside
> DCHECKS. ..."

DOH. LGTM.

[asargent_no_longer_on_chrome, 2014-05-16 01:49:09]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2014-05-16 01:49:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asargent@chromium.org/288273004/40001

[commit-bot: I haz the power, 2014-05-16 05:12:21]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[asargent_no_longer_on_chrome, 2014-05-16 05:22:59]

Committed patchset #3 manually as r270937 (presubmit successful).