This also adds ImmutableStringBuilder class, which can be used to
build ImmutableStrings in place without extra allocations if the
maximum length is known in advance.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I4dfb78adeb0cffcfad0d25753fb8063466012c92
Reviewed-on: https://chromium-review.googlesource.com/886362
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>