Moving Blink from hidden values to private symbols almost doubled the
rate of API calls, as the private symbols API increases the call-depth
scope while the hidden values one didn't.

I manually checked that this fixes the regression on the long running
gmail benchmark but doesn't add new memory reducer GCs on the infinite
scrolling benchmark

BUG=chromium:561325
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1472193003

Cr-Commit-Position: refs/heads/master@{#32279}