HeapSnapshot: speed-up consStringName. It is quite slow. About 6 sec for 100k consString chain.

I just inlined everything and got 3 times boost.
Unfortunately I can't compare strings by indexes because our strings array has duplicates.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23513051

git-svn-id: svn://svn.chromium.org/blink/trunk@157765 bbb929c8-8fbe-4397-9dbb-9b2b20218538