When doing advance at the start of an unanchored unicode regexp,
we do not have to care about surrogate pairs. If we actually advance
into the middle of a surrogate pair, the only choice is to also
consume trail surrogate as nothing else can match from there.

This reduces the emitted code slightly. By not having choice in the
loop, we do not have to push backtrack onto the stack, preventing
stack overflow.

R=erik.corry@gmail.com, erikcorry@chromium.org

Review URL: https://codereview.chromium.org/1676293003

Cr-Commit-Position: refs/heads/master@{#33838}