Avoid some unneeded malloc in GridIterator

The issue was that the local variables didn't have the same type
(Vector<RenderBox*>) as that of the grid representation
(Vector<RenderBox*, 1>), which forced a hidden copy. The fix is
to add the missing typedefs to avoid type mismatchs in the future.

This caused PerformanceTests/Layout/auto-grid-lots-of-data.html to
severely regress after crrev.com/22867006. On a Z620, this improves
the test from 83.5 run/s to 88 run/s (a 5% improvement).

BUG=275745

Review URL: https://chromiumcodereview.appspot.com/23137008

git-svn-id: svn://svn.chromium.org/blink/trunk@156416 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a layout test for testing snapshot file reference issue

The last file reference to a snapshot file can be lost if
a File reference is being GC'ed while second .file() is
being called to create another snapshot.

Chromium-side patch: https://codereview.chromium.org/22854018

BUG=275224
TEST=fast/filesystem/snapshot-file-with-gc.html

Review URL: https://chromiumcodereview.appspot.com/23179006

git-svn-id: svn://svn.chromium.org/blink/trunk@156417 bbb929c8-8fbe-4397-9dbb-9b2b20218538