Preven OOB memory access in chrome_pdf::AlphaBlend().

Preven OOB memory access in chrome_pdf::AlphaBlend().

Similar to bug 398384, but perform the same bounds check we introduced to 
chrome_pdf::CopyImage() in the chrome_pdf::AlphaBlend() function.

Also change the return value from bool to void, since AlphaBlend() always
returns true, and no-one was checking the return value anyways.
 
BUG=384891
Committed: https://crrev.com/8f07983fb62fb48fc8298b150e4548f92a374da1
Cr-Commit-Position: refs/heads/master@{#293454}

[Tom Sepez, 2014-09-04 22:17:55]

tsepez@chromium.org changed reviewers:
+ gene@chromium.org

[Tom Sepez, 2014-09-04 22:18:08]

gene, please review. Thanks.

[gene, 2014-09-05 00:51:13]

lgtm

[Tom Sepez, 2014-09-05 01:25:25]

The CQ bit was checked by tsepez@chromium.org

[commit-bot: I haz the power, 2014-09-05 01:28:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tsepez@chromium.org/544863002/1

[commit-bot: I haz the power, 2014-09-05 05:43:10]

Committed patchset #1 (id:1) as 0807a3021aaa309d9a8473c2b58238a061e64a35

[commit-bot: I haz the power, 2014-09-10 03:37:18]

Patchset 1 (id:??) landed as
https://crrev.com/8f07983fb62fb48fc8298b150e4548f92a374da1
Cr-Commit-Position: refs/heads/master@{#293454}