Add more comments to base/values.h.

For every FooPath() method, mention using FooKey() when there is only a
single component in the path. Some of the deprecation comments elsewhere
in the file recommends using FooPath(), so those trying to learn the new
base::Value API may not notice the existance of FooKey().

Also fit some nits in the file.

Change-Id: I025dd64972d4c84936d506d8aced28a0d04e7fe9
Reviewed-on: https://chromium-review.googlesource.com/738799
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512037}

• base/values.h[diff]