Some accessors requires little to no computation at all, its result can be
cached in a private property, avoiding the call overhead.
Calls to the getter are translated into a cheap property load.

Follow-on to crrev.com/2347523003, from peterssen@google.com

BUG=chromium:634276, v8:5548

Review-Url: https://codereview.chromium.org/2405213002
Cr-Commit-Position: refs/heads/master@{#40765}