Fix console messages in SRI errors.

Fix console messages in SRI errors.

The Subresource Integrity checker logs error messages if no matching
integrity attribute is found, including a suggested SHA 256 digest.
Unfortunately, when calculating the digest, a previously calculated hash
value occasionally was still lying around in the digest vector, putting
several extraneous characters into the console.. This clears the Vector
before calculating the digest, thus removing any extra data that may be
in the Vector from earlier.

TBR=mkwst@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196364

[jww, 2015-06-02 18:45:10]

The CQ bit was checked by jww@chromium.org

[commit-bot: I haz the power, 2015-06-02 18:45:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1166773002/1

[commit-bot: I haz the power, 2015-06-02 22:06:57]

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

[commit-bot: I haz the power, 2015-06-02 22:06:59]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/64845)

[jww, 2015-06-03 01:18:49]

The CQ bit was checked by jww@chromium.org

[commit-bot: I haz the power, 2015-06-03 01:19:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1166773002/1

[commit-bot: I haz the power, 2015-06-03 02:49:24]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196364