Change HashMap declaration in ResourceLoadPriorityOptimizer to use UnsignedWithZeroKeyHashTraits.

Key values of 0 aren't allowed in HashMap with default traits, and we'll hit an assert in HashTable since
we just use the resource's identifier as key in the table.

BUG=369412

Review URL: https://codereview.chromium.org/271323002

git-svn-id: svn://svn.chromium.org/blink/trunk@173912 bbb929c8-8fbe-4397-9dbb-9b2b20218538