Adds @UsedByReflection to the CronetUrlRequestContext constructor

Adds @UsedByReflection to the CronetUrlRequestContext constructor. This is necessary for UrlRequestContext to instantiate this class after proguarding.

BUG=

Committed: https://crrev.com/527f8a00689866ce3ac84a9fc75a579af73fe735
Cr-Commit-Position: refs/heads/master@{#322208}

[groakley, 2015-03-25 18:45:25]

The CQ bit was checked by groakley@google.com

[groakley, 2015-03-25 18:45:26]

The CQ bit was unchecked by groakley@google.com

[groakley, 2015-03-25 18:48:39]

groakley@google.com changed reviewers:
+ mef@chromium.org

[mef, 2015-03-25 19:13:29]

lgtm, thanks for catching this!

[groakley, 2015-03-25 19:15:27]

The CQ bit was checked by groakley@google.com

[commit-bot: I haz the power, 2015-03-25 19:15:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1018213007/1

[commit-bot: I haz the power, 2015-03-25 19:21:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-25 19:23:22]

Patchset 1 (id:??) landed as
https://crrev.com/527f8a00689866ce3ac84a9fc75a579af73fe735
Cr-Commit-Position: refs/heads/master@{#322208}