This was causing an assertion failure in updateStorageLevel when the
storage didn't contain the requested level.  Gate the call behind an
isValidLevel check.

TEST=conformance/misc/type-conversion-test.html

BUG=angleproject:1815

Change-Id: Iadf46d232d5a117ae1ae74c6e71677b1e06a5321
Reviewed-on: https://chromium-review.googlesource.com/572705Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>