Implement stub of Blink API to check for enabled experiments

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
Committed: https://crrev.com/153f0dd478de52c46f77abeb1fa8a94cbd89560f
Cr-Commit-Position: refs/heads/master@{#354887}

[chasej, 2015-10-14 23:17:51]

chasej@chromium.org changed reviewers:
+ mek@chromium.org

[chasej, 2015-10-14 23:17:52]

mek, please take a look.

[Marijn Kruisselbrink, 2015-10-15 00:34:08]

lgtm

[chasej, 2015-10-16 17:54:06]

chasej@chromium.org changed reviewers:
+ rbyers@chromium.org

[chasej, 2015-10-16 17:54:06]

rbyers, please take a look.

[Rick Byers, 2015-10-19 20:09:09]

LGTM

[chasej, 2015-10-19 20:20:19]

The CQ bit was checked by chasej@chromium.org

[commit-bot: I haz the power, 2015-10-19 20:21:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1404733003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1404733003/1

[commit-bot: I haz the power, 2015-10-19 22:22:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-19 22:23:35]

Patchset 1 (id:??) landed as
https://crrev.com/153f0dd478de52c46f77abeb1fa8a94cbd89560f
Cr-Commit-Position: refs/heads/master@{#354887}