[CRD iOS] Change audio session category to Ambient

AVAudioSessionCategoryAmbient works basically like
AVAudioSessionCategoryPlayback+AVAudioSessionCategoryOptionMixWithOthers,
except that it allows the audio to be muted by the silent switch.
This a more proper category to use because the app can still work with no
sound. Only apps like music player that requires audio playback when
silent switch is on should use the AVAudioSessionCategoryPlayback
category.

Bug: 868088
Change-Id: I56163a221d878f67363aa8cff7a0151e9602e1c1
Reviewed-on: https://chromium-review.googlesource.com/1196020
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587393}
1 file changed