NVS: Enable "apply_to_external_hits" by default

Dev channel experiments on the apply_to_external_hits feature parameter
of HttpCacheNoVarySearch demonstrated that it has the expected effect of
reducing the number of new cache entries that are evicted from the
blockfile cache, but has no statistically significant effects on other
metrics. As such, further experiments on this parameter are not
interesting, so just turn on the feature by default.

BUG=382394774

Change-Id: Id74d98ba213b3c1c65fbac06f9bffe50e71748c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6929841
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1514143}