Revert to CHECK when null entries appear in the throttler map.
Revert to CHECK when null entries appear in the throttler map.

This was always the plan for after the M11 branch point. A recent
change (pkasting's r77258) fixed a memory corruption issue, possibly
the same as what we're seeing, but hard to verify unless in the wild.

BUG=71721
TEST=none

Review URL: http://codereview.chromium.org/6680005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@77856 0039d316-1c4b-4281-b951-d872f2087c98

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