Change ASSERTs for failures to find items in a hashmaps to ASSERT_WITH_SECURITY_IMPLICATIONs. This results in an out-of-bounds memory access as the iterator is equal to hash map's end and we try to read/write to that location.

R=inferno@chromium.org
BUG=318738

Review URL: https://codereview.chromium.org/59883015

git-svn-id: svn://svn.chromium.org/blink/trunk@162224 bbb929c8-8fbe-4397-9dbb-9b2b20218538