Make ServiceRuntime::LoadModule asynchornize-able.
This is just a refactoring to prepare making trusted plugin channel
an async IPC. Currently, we're using sync SRPC call. This CL still
keeps using it as is, but makes the function interfaces friendly
to async calls.
TEST=run browser_tests --gtest_filter=NaCl* locally, and trybots.
BUG=333950
R=teravest@chromium.org
Review URL: https://codereview.chromium.org/
326593002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276268 0039d316-1c4b-4281-b951-d872f2087c98