This adds a missing conversion of time zone names to string primitives
before case conversion and regular expression matching are applied.

This is in sync with ECMA-402, section 12.1.1.1, step 16(a).

R=jochen@chromium.org
TEST=mjsunit/regress/regress-crbug-627935
BUG=chromium:627935

Review-Url: https://codereview.chromium.org/2143003005
Cr-Commit-Position: refs/heads/master@{#37757}