Rename ObjectAliveTrait::isAlive() to isHeapObjectAlive().

This method is only invoked by Visitor::isAlive() and never explicitly
declared in "user code", so switch to a name less likely to conflict
with what an object might otherwise provide.

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/633463002

git-svn-id: svn://svn.chromium.org/blink/trunk@183282 bbb929c8-8fbe-4397-9dbb-9b2b20218538