This change rewrites regexps like (ab|ac|z|ad|ae|af) into (a[b-f]|z).  We can only reorder disjunctions like this for case-dependent regexps.  For case-independent regexps, the disjunctions should be pre-sorted for best results.

R=yangguo@chromium.org
BUG=chromium:482998
LOG=n

Review URL: https://codereview.chromium.org/1180433003

Cr-Commit-Position: refs/heads/master@{#28902}