Oilpan: fix (MSVC) build after r183241.

Rename WebFrame::isAlive(Visitor*, WebFrame*) to isFrameAlive(). Using
isAlive() is problematic wrt IsGarbageCollectedMixin<T>, which
syntactically probes for isAlive().

(MSVC is more eager about template instantiation and the probing for the
private WebFrame::isAlive() method is disallowed.)

R=haraken,tkent
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/628063003

git-svn-id: svn://svn.chromium.org/blink/trunk@183247 bbb929c8-8fbe-4397-9dbb-9b2b20218538