Fix the GlobalTitleFail debug notice for HTMLButtonFields

These occur when buttonlabel-message is set. HTMLButtonField
parses the message in its constructor, but at that point the
context (provided by the form field by HTMLForm::$mParent) is
not yet available. The constructor of HTMLForm assigns $mParent,
but that constructor is only called after the button label
message is parsed.

Bug: T201497
Change-Id: I021c9ecf3bc934f2cf55ec100e799c1c12e7bc01