Use an empty origin as source origin when posting messages from android_webview

There is no point of keeping source_origin as a parameter in postMessageTo* methods. It is not a parameter to expose to the source app in a webview API, and it is not a value that will change from one request to another, so for the simplicity's sake, remove it from java side completely. At the moment, I am not fully sure what a good value is so leaving it as a empty domain for now.

BUG=393291
Committed: https://crrev.com/274fd4ebccc314bd99dbf328d614d4e615f69ce3
Cr-Commit-Position: refs/heads/master@{#316295}

[sgurun-gerrit only, 2015-02-13 20:17:24]

sgurun@chromium.org changed reviewers:
+ hush@chromium.org, mnaganov@google.com

[sgurun-gerrit only, 2015-02-13 20:18:10]

ptal, thanks.

[hush (inactive), 2015-02-13 21:14:00]

lgtm

[sgurun-gerrit only, 2015-02-13 21:14:39]

The CQ bit was checked by sgurun@chromium.org

[commit-bot: I haz the power, 2015-02-13 21:14:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/923913004/1

[commit-bot: I haz the power, 2015-02-13 21:36:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-13 21:37:14]

Patchset 1 (id:??) landed as
https://crrev.com/274fd4ebccc314bd99dbf328d614d4e615f69ce3
Cr-Commit-Position: refs/heads/master@{#316295}