Omit histogram bucket min/max in some cases to optimize UMA size.

Also updates proto file comments and marks bucket_index as
deprecated.

BUG=288750
TEST=New unit test.

Review URL: https://codereview.chromium.org/25611005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227115 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227115