Remove comparisons of this with NULL in debugging code.

All in-code calls of these functions already check for null. While this might
be null when calling from a debugger, the debugger will give one a fairly clear
message in that case anyhow -- and calling methods on null objects is undefined
behavior, so the compiler might optimize the check away anyhow.

BUG=381910

Review URL: https://codereview.chromium.org/350113003

git-svn-id: svn://svn.chromium.org/blink/trunk@177022 bbb929c8-8fbe-4397-9dbb-9b2b20218538