Implement stub of Blink API to check for enabled experiments

Allows APIs implemented in Blink to check (from native code), if the
API is enabled for the current context.  Stub implementation will
always return that the API is disabled.

API is synchronous so it can be called from API implementations that are
synchronous.

BUG=543140

Review URL: https://codereview.chromium.org/1404733003

Cr-Commit-Position: refs/heads/master@{#354887}
4 files changed