Perform a sizet->int checked_cast in WebURLLoaderImpl::OnReceiveWebBlobData.

Perform a sizet->int checked_cast in WebURLLoaderImpl::OnReceiveWebBlobData.

Fixes an error on my Windows build:
d:\src\chrome-git\src\components\html_viewer\web_url_loader_impl.cc(254) : error C2220: warning treated as error - no 'object' file generated
d:\src\chrome-git\src\components\html_viewer\web_url_loader_impl.cc(254) : warning C4267: 'argument' : conversion from 'size_t' to 'int', possible loss of data

BUG=NONE
TEST=Windows html_viewer build succeeds.
R=erg@chromium.org
TBR=sky@chromium.org

Committed: https://crrev.com/7142727598a3ce06b0e636004dfb1646e677b65d
Cr-Commit-Position: refs/heads/master@{#326898}

[msw, 2015-04-24 21:27:48]

Hey Elliot, please take a look; thanks!

[Elliot Glaysher, 2015-04-24 21:33:14]

lgtm

[msw, 2015-04-24 21:39:10]

msw@chromium.org changed reviewers:
+ sky@chromium.org

[msw, 2015-04-24 21:39:29]

TBR'ing Scott for OWNERS; I also fixed a typo.

[msw, 2015-04-24 21:39:37]

The CQ bit was checked by msw@chromium.org

[msw, 2015-04-24 21:39:37]

The patchset sent to the CQ was uploaded after l-g-t-m from erg@chromium.org
Link to the patchset: https://codereview.chromium.org/1096243004/#ps20001
(title: "Fix comment typo.")

[commit-bot: I haz the power, 2015-04-24 21:41:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1096243004/20001

[commit-bot: I haz the power, 2015-04-24 22:05:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-24 22:06:46]

Patchset 2 (id:??) landed as
https://crrev.com/7142727598a3ce06b0e636004dfb1646e677b65d
Cr-Commit-Position: refs/heads/master@{#326898}