[Chromecast] Initialize CastMediaShlib on caller's thread
[Chromecast] Initialize CastMediaShlib on caller's thread

Could fix the race condition where AvSettingsShlib is Initialized first
before CastMediaShlib.

BUG=internal b/24585473

Review URL: https://codereview.chromium.org/1385863002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352328}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk