TechCrunch should use 600 kB less memory

Before this CL, we could end up with duplicate copies of JavaScript code if the
CachedScript was evicted from the MemoryCache but V8 retained the underlying
String and we loaded the script again. Similarly, we could end up with
duplicates if a page loads an identical script from two distinct URLs.

This CL changes the backing store for CachedScript to an AtomicString to
coalesce these copies. We already coalesce inline scripts for the same reason.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/21133009

git-svn-id: svn://svn.chromium.org/blink/trunk@155200 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Replace RenderArena with PartitionAlloc.

We believe this is memory positive, a very minor binary size saving, and perf neutral; see the bug for measurements. It's definitely a code clean-up. Perf bots will be carefully monitored after the land.

(This CL is mostly a subset of previously-reviewed https://codereview.chromium.org/16896019/, which we tried to land in a few pieces, to the limited extent possible.)

BUG=252057
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/20231002

git-svn-id: svn://svn.chromium.org/blink/trunk@155202 bbb929c8-8fbe-4397-9dbb-9b2b20218538