Refine FilterEffect::createTransparentBlack

Refine FilterEffect::createTransparentBlack

Use an image filter that will produce a region of pixels rather than one
that won't.
Fixes issue with feConvolveMatrix error handling with -webkit-filter.

BUG=533457
Committed: https://crrev.com/eb684ee648153e593d4ace3e0eb4612b205d29f0
Cr-Commit-Position: refs/heads/master@{#351658}

[fs, 2015-09-30 12:26:33]

fs@opera.com changed reviewers:
+ senorblanco@chromium.org

[Stephen White, 2015-09-30 18:20:58]

LGTM

[fs, 2015-09-30 22:05:50]

The CQ bit was checked by fs@opera.com

[commit-bot: I haz the power, 2015-09-30 22:06:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1380733002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1380733002/1

[commit-bot: I haz the power, 2015-09-30 22:15:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-30 22:17:03]

Patchset 1 (id:??) landed as
https://crrev.com/eb684ee648153e593d4ace3e0eb4612b205d29f0
Cr-Commit-Position: refs/heads/master@{#351658}