Close a loophole in extension content verification

Close a loophole in extension content verification

If you both deleted the computed_hashes.json and modified the
verified_contents.json so that the signature failed, we'd never fix it.
This patch changes things so that we validate the verified_contents.json
instead of just assuming that the file existing means it's contents are
ok.

BUG=402727
R=rockot@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=289567

[asargent_no_longer_on_chrome, 2014-08-13 23:10:28]

(still working on a unit test for this, but wanted to get the change in ASAP)

[Ken Rockot(use gerrit already), 2014-08-14 04:50:36]

lgtm

[asargent_no_longer_on_chrome, 2014-08-14 05:26:26]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2014-08-14 05:28:54]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asargent@chromium.org/465333003/40001

[commit-bot: I haz the power, 2014-08-14 11:33:25]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[commit-bot: I haz the power, 2014-08-14 12:02:44]

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

[commit-bot: I haz the power, 2014-08-14 12:02:45]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[asargent_no_longer_on_chrome, 2014-08-14 14:42:37]

Committed patchset #3 manually as 289567 (presubmit successful).