Avoid buffer overflow in PageSerializerTest.XMLDeclaration

Specify length when creating String from pointer, to avoid heap-buffer-overflow detected by AddressSanitizer.

BUG=313988

Review URL: https://codereview.chromium.org/54373012

git-svn-id: svn://svn.chromium.org/blink/trunk@161224 bbb929c8-8fbe-4397-9dbb-9b2b20218538