trunk-tracking: update from r3715 to r3736

NOTE: This passes tests only in non-valgrind non-native-fetcher mode right now.  JeffTK promised to look at it separately to see what's going on as there were "interesting" changes to ngx_pagespeed.cc and nginx_system_test.sh

r3726:
Updated closure compiler flags for static JS files.

r3729:
Centralize parsing of FetchHttps in SystemRewriteOptions
so ngx_pagespeed can get it too.

To keep the helpful error_message from SerfUrlAsyncFetcher, wire it
through RewriteOptions as a new-fangled error_detail.

r3735:
Follow-up changes for downstream caching integration with beaconing dependent
filters: If a downstream cache rebeaconing key is configured, we should
instrument the page only if the key present in the PS-ShouldBeacon header
matches the one in the configuration. This allows us to send no-cache headers
for anything that carries the right beaconing key, and continue to send out the
original cache control headers in other cases where downstream caching is
enabled.