XSS Auditor: Block by default.

This patch changes the default behavior of the XSS auditor from "filter"
to "block". It also fixes a bug exposed by this change: blocking a page
in the middle of parsing/processing `document.write()` crashes the
renderer due to a null deref.

The vast majority of this change is changing layout tests to specify
filtering behavior rather than default behavior.

Intent to Ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/aZsNygF84JM/86EbD_q0CAAJ

BUG=654794

Review-Url: https://codereview.chromium.org/2524013002
Cr-Commit-Position: refs/heads/master@{#434392}
229 files changed