Add an explicit bool conversion to scoped_refptr<T>.

Add an explicit bool conversion to scoped_refptr<T>.

This wasn't feasible with the Testable trick, because that resulted in
an ambiguous overload. Enabling this explicit conversion now will result
in a lot less churn when converting Android, iOS, and Windows.

BUG=110610

[dcheng, 2014-09-26 00:09:03]

dcheng@chromium.org changed reviewers:
+ ajwong@chromium.org, jamesr@chromium.org, thakis@chromium.org

[dcheng, 2014-09-26 00:09:03]

Assuming this actually passes the trybots and really is an unambiguous
conversion...

I know explicit conversions aren't actually permitted yet, but this will greatly
reduce the amount of churn for future CLs. I know that the intermediate state is
quite confusing/annoying for a lot of people, so I think it's worth making a
temporary exception here (especially since all the other smart pointers types
can already be used in boolean tests!)

[jamesr, 2014-09-26 05:36:43]

e:\b\build\slave\win\build\src\chrome\installer\util\wmi.cc(72) :error C2666:
'operator ==' : 2 overloads have similar conversions
       
e:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\include\shared\guiddef.h(192):
could be 'bool operator ==(const GUID &,const GUID &)'
        while trying to match the argument list '(int,
base::win::ScopedComPtr<IWbemClassObject,&
_GUID_dc12a681_737f_11cf_884d_00aa004b2e24>)'
ninja: build stopped: subcommand failed.

huh, is this from this patch? not clear to me what's going on, exactly.

[jamesr, 2014-09-26 05:37:27]

Ah, ScopedComPtr inherits from scoped_refptr.  Maybe there's a way to patch up
that subclass specifically?

[dcheng, 2014-09-26 07:21:32]

The good news is the ambiguous comparison overloads for scoped_refptr are all
comparisons to NULL (as far as I can tell).

The bad news is I think this means the explicit conversion operator might be
busted for the Windows toolchain?!
http://stackoverflow.com/questions/24311437/msvc-2013-explicit-operator-bool and
http://stackoverflow.com/questions/20498142/visual-studio-2013-explicit-keywo...
support this theory.