Trivial changes to default Vector sizing in the shadow area.

This enables us to lower the default size expansion of Vector from 16 to 4
without taking a performance hit on
ShadowDOM/DistributionWithMultipleShadowRoots

These changes may even make performance faster than before the default Vector
size change, but the bots will have to be the judge of that.

BUG=286696

Review URL: https://chromiumcodereview.appspot.com/23766020

git-svn-id: svn://svn.chromium.org/blink/trunk@157725 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web MIDI: add kouhei@ to owner files for the API

BUG=none
TEST=none
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/23875020

git-svn-id: svn://svn.chromium.org/blink/trunk@157726 bbb929c8-8fbe-4397-9dbb-9b2b20218538