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

Review URL: https://codereview.chromium.org/1208703002

Cr-Commit-Position: refs/heads/master@{#336028}
1 file changed