Cap allocation for ImmutableStylePropertySet.

The m_arraySize will be capped to MaxArraySize, so it is unnecessary to
allocate room for more than MaxArraySize properties.

Leftover from https://codereview.chromium.org/22917005/

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23708049

git-svn-id: svn://svn.chromium.org/blink/trunk@158093 bbb929c8-8fbe-4397-9dbb-9b2b20218538