U+feff is the UTF BOM but if it occurs inside the text, it's a "zero-width
no-break space". However, the UTF-8 decoder in script streaming still thought
it's a BOM and skipped it. The correct way to handle it would be to create a
U+feff code point instead - the Scanner will then handle it as whitespace.

This is a discrepancy between the Blink UTF-8 decoder and the V8 UTF-8 decoder,
and caused the source positions be off by one. This bug went unnoticed, since
normally off-by-one in this situation doesn't make the code to break.

BUG=chromium:758508,chromium:758236

Change-Id: Ib92a3ee65c402e21b77e42537db2a021cff55379
Reviewed-on: https://chromium-review.googlesource.com/632096Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47583}