cc : Add DCHECKs to ensure we have valid proprety tree indices at commit

There has been a spike in crash reports because of invalid proeprty tree
indices recently(crbug.com/712586, crbug.com/708951). This CL adds some
DCHECKs for sanity checking.
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2829853009
Cr-Commit-Position: refs/heads/master@{#467204}
5 files changed