Fix name of mojo test source file so it gets compiled.

Fix name of mojo test source file so it gets compiled.

The .cc extension to js_to_cpp_tests.cc was omitted so the file has
never been compiled and therefore the tests never run when using
gyp files. The fix is to add the .cc.

It is unfortunate that gyp silently ignores missing files.

R=ben@chromium.org
BUG=504157
Committed: https://crrev.com/590d6351fe0aa496622c7a8f9f4ada6851c5d892
Cr-Commit-Position: refs/heads/master@{#336028}

[brucedawson, 2015-06-24 22:52:30]

I get a few errors when I run mojo_js_integration_tests (on Windows) without
this fix and a bunch more (crashes) with this fix. I'm not sure if that means
that my test environment is not set up correctly (Windows!) or if these tests
have not been exercised and are genuinely broken.

[Ben Goodger (Google), 2015-06-24 23:06:21]

lgtm

[brucedawson, 2015-06-24 23:08:15]

The CQ bit was checked by brucedawson@chromium.org

[commit-bot: I haz the power, 2015-06-24 23:10:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1208703002/1

[commit-bot: I haz the power, 2015-06-24 23:19:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-24 23:20:15]

Patchset 1 (id:??) landed as
https://crrev.com/590d6351fe0aa496622c7a8f9f4ada6851c5d892
Cr-Commit-Position: refs/heads/master@{#336028}