Escaping used to happen lazily, implemented in an accessor property.
However, native implementation of RegExp methods use .source as well.
This leads to performance regressions. Now we do it eagerly instead.

R=jkummerow@chromium.org
BUG=chromium:436447
LOG=N

Review URL: https://codereview.chromium.org/1070093002

Cr-Commit-Position: refs/heads/master@{#27705}