Small refactor to MetricsLog stability functionality.

This CL simplifies some MetricsLog APIs, consolidating
RecordEnvironment and RecordEnvironmentProto as well as
RecordIncrementalStabilityElements and WriteStabilityElement.

This reduces the MetricsLog API surface, simplifying things
for some upcoming changes I'm planning to make (recording
startup stability stats separately from startup histograms
and profiler data).

No changes to logic/functionality.

BUG=312733, 109818
TEST=Existing unit tests.
TBR=grt@chromium.org

Review URL: https://codereview.chromium.org/69603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236551 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236551