Move ENABLE_MEDIA_CAPTURE to a runtime flag

The effort needs 3 steps:
1. Create the runtime flag interface in blink;
2. Enable the flag for Android from chromium even it does not do anything in blink;
3. Replace compile time guard with runtime flag in blink and remove the build time flag

BUG=388093

Review URL: https://codereview.chromium.org/350653008

git-svn-id: svn://svn.chromium.org/blink/trunk@176812 bbb929c8-8fbe-4397-9dbb-9b2b20218538