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When the Blink cache produces conditional requests, the Chrome cache
always sends them to the origin server without attempting to serve them
from the cache. Add a histogram to find out how common conditional
requests from Blink are, and how many are hitting the network
even though a usable Chrome cache entry exists.

BUG=407968
TEST=manual inspection of about:histograms

Review URL: https://codereview.chromium.org/531693003

Cr-Commit-Position: refs/heads/master@{#293934}