[Chromecast] Use base::FeatureList to control features.

In Chromium, Finch-enabled features are controlled through base::FeatureList,
a class which abstracts the experiment framework and developer overrides
from client code. Though Chromecast's experiment framework is fundamentally
different (in that it is server-driven) Cast builds can still make use of
this class. Introduce some utilities to help.

At boot-up, the pref store will be queried for experiment configs, which
were cached to disk on the most recent config fetch from the last boot
cycle. If a developer overrides these features from the command line,
that value takes precedence. These features will be used to initialize
base::FeatureList, which can then be statically queried from any client
code that depends on //base.

This patch does not actually introduce or convert any existing features
to use this framework.

BUG=714291
BUG= internal b/35424335

Review-Url: https://codereview.chromium.org/2825873002
Cr-Commit-Position: refs/heads/master@{#466507}