Add helper to validate JPEG decode subsampling factors

Add helper to validate JPEG subsampling factors

Use it to turn the invalid horizontal/vertical subsampling factor 3 to a
1, which is fine for both the YUV decoding path (it won't turn on if any 
of the component subsamplings is 3) and the normal JPEG decode path.

TEST=Covered by the test added in r192900
BUG=398235
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=192980

[Noel Gordon, 2015-03-29 09:45:55]

noel@chromium.org changed reviewers:
+ reveman@chromium.org, sugoi@chromium.org

[Noel Gordon, 2015-03-29 09:51:20]

Note that subsampling factors are limited to [1..4] by the JPEG decode library,
producing a failure if they are out of bounds. See for example: 

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjpe...

[Noel Gordon, 2015-04-01 05:16:07]

Test harness setup for this now -> we can go anyway we like in this CL with that
safety net in place viz., map 3->1, or deny JPEG with any 3, yada yada. My view
is go this CL... 

...so Alexis, could you review please?   And please check that I din't break YUV
decoding :)

[sugoi1, 2015-04-01 12:11:51]

On 2015/04/01 05:16:07, noel gordon wrote:
> Test harness setup for this now -> we can go anyway we like in this CL with
that
> safety net in place viz., map 3->1, or deny JPEG with any 3, yada yada. My
view
> is go this CL... 
> 
> ...so Alexis, could you review please?   And please check that I din't break
YUV
> decoding :)

It will not affect YUV decoding since this happens after the call to
yuvSubsampling(), so LGTM for that part.
I don't fully understand why changing subsampling from 3 to 1 works for the
regular JPEG decode though, since I don't know about those specific
implementation details on the regular JPEG decode path.

[Noel Gordon, 2015-04-01 16:34:45]

On 2015/04/01 12:11:51, sugoi1 wrote:
> On 2015/04/01 05:16:07, noel gordon wrote:
> > Test harness setup for this now -> we can go anyway we like in this CL with
> that
> > safety net in place viz., map 3->1, or deny JPEG with any 3, yada yada. My
> view
> > is go this CL... 
> > 
> > ...so Alexis, could you review please?   And please check that I din't break
> YUV
> > decoding :)
> 
> It will not affect YUV decoding since this happens after the call to
> yuvSubsampling(), so LGTM for that part.

Thanks for checking.

> I don't fully understand why changing subsampling from 3 to 1 works for the
> regular JPEG decode though, since I don't know about those specific
> implementation details on the regular JPEG decode path.

Test case on the bug produces stable pixels for me with this change. Still
awaiting feedback on the bug about 3, but for now, let's run this change through
our other systems and see if they complain.

[Noel Gordon, 2015-04-01 16:34:51]

The CQ bit was checked by noel@chromium.org

[commit-bot: I haz the power, 2015-04-01 16:35:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039503003/1

[commit-bot: I haz the power, 2015-04-01 16:35:17]

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

[commit-bot: I haz the power, 2015-04-01 16:35:18]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Noel Gordon, 2015-04-01 16:36:00]

noel@chromium.org changed reviewers:
+ pdr@chromium.org

[Noel Gordon, 2015-04-01 16:36:40]

On 2015/04/01 16:35:18, I haz the power (commit-bot) wrote:
> No LGTM from a valid reviewer yet. Only full committers are accepted.

pdr@ rubber stamp please?

[pdr., 2015-04-01 18:08:37]

On 2015/04/01 at 16:36:40, noel wrote:
> On 2015/04/01 16:35:18, I haz the power (commit-bot) wrote:
> > No LGTM from a valid reviewer yet. Only full committers are accepted.
> 
> pdr@ rubber stamp please?

LGTM but I'd really like a test for this. I don't have access to the linked bug
so I may not have the full context.

[Noel Gordon, 2015-04-01 23:57:14]

On 2015/04/01 18:08:37, pdr wrote:
> On 2015/04/01 at 16:36:40, noel wrote:
> > On 2015/04/01 16:35:18, I haz the power (commit-bot) wrote:
> > > No LGTM from a valid reviewer yet. Only full committers are accepted.
> > 
> > pdr@ rubber stamp please?
> 
> LGTM but I'd really like a test for this. I don't have access to the linked
bug
> so I may not have the full context.

Already added a test (r192900).

[Noel Gordon, 2015-04-01 23:58:25]

The CQ bit was checked by noel@chromium.org

[commit-bot: I haz the power, 2015-04-01 23:59:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039503003/1

[commit-bot: I haz the power, 2015-04-02 01:44:51]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=192980

[Noel Gordon, 2015-04-08 05:12:36]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1069083003/ by noel@chromium.org.

The reason for reverting is: Per the bug, Factor 3 images are acceptable to
libjpeg6b and users can even create them using its well-known cjpeg tool. .