Remove the canary guard value at the start of the partition page.

Now that we're about to land out-of-line metadata, it is not required.
Furthermore, getting rid of it is instrumental to being able to shrink
sizeof(PartitionPage) to 32 in a follow-on change.

BUG=324960

Review URL: https://codereview.chromium.org/99883005

git-svn-id: svn://svn.chromium.org/blink/trunk@163114 bbb929c8-8fbe-4397-9dbb-9b2b20218538