Add enable*Features functions in preparation for using them in content.

Add enable*Features functions in preparation for using them in content.

I've manually verified that these are the exact set of flags that
RenderThreadImpl::InitializeWebKit() sets.  Once this lands and
rolls into chromium, I will change RenderThreadImpl to use these
functions and then we can autogenerate this list from
RuntimeEnabledFeatures.in instead of having it hard-coded.

Note that these lists do not take ports into account, that's
currently still a content-layer concern.  Eventually perhaps
we should make RuntimeEnabledFeatures aware of how "stable"
on android may be different from "stable" on other platforms
but for now I've left that in content.

BUG=237740
R=dpranke@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=149671

[Dirk Pranke, 2013-05-03 22:24:00]

lgtm

[eseidel, 2013-05-03 22:31:50]

Now I just need to get jamesr, tkent, abarth, darin, or dglazkov to approve the
header change.

[dglazkov, 2013-05-03 23:48:01]

LGTM

[eseidel, 2013-05-03 23:55:51]

Committed patchset #3 manually as r149671 (presubmit successful).