Do not include Assertions.h within namespace WebKit.

Including Assertions.h leads to __darwin_ssize_t being
defined inside namespace WebKit but not outside that namespace.
This is because certain headers will not be pulled in again
later on in the same compilation unit.

This bug is caused by: r155495

Review URL: https://chromiumcodereview.appspot.com/23052005

git-svn-id: svn://svn.chromium.org/blink/trunk@156169 bbb929c8-8fbe-4397-9dbb-9b2b20218538