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Every call to base::Bind results in a new class declaration. Since
base::internal::BindState is a virtual class, this also results in a
bespoke vtable being defined. And that's a lot of vtables. Like 1MB
worth of vtables, or 1% of Chrome's binary size.

However, this vtable's sole purpose is for RefCountedThreadSafe::
Release(). Release() _does_ need to know precisely how to delete the
BindState, but it doesn't need a vtable to do it. All it needs is a
function pointer.

This CL de-virtualizes base::BindState and instead moves the vtable
pointer (which is merely an array holding the destructor) to a function
pointer that contains the destructor instead. Since BindState previously
contained a pointer-to-vtable the net memory effect of having the
function pointer instead should be offset.

And of course a 1MB binary size drop also means reduced memory
requirements for the shared text-segment pages.

Locally produced numbers for ChromiumFramework on Mac:
pre-strip:
 - before: 192,031kB
 - after: 185,800kB
post-strip:
 - before: 101,353kB
 - after: 100,353kB

BUG=486594

Review URL: https://codereview.chromium.org/1129353003

Cr-Commit-Position: refs/heads/master@{#329818}