Use explicit operator bool in scoped_refptr.
Use explicit operator bool in scoped_refptr.

Previously the "Safe Bool Idiom" was incorrectly
implemented using a typedef ... Testable that leaked
the raw pointer of scoped_refptr.

Code relying on that side effect has been corrected
in https://codereview.chromium.org/1958823002/

The C++11 explicit conversion operator feature
is now available and this hack is no longer required,
and usage will be enforced to not inadvertently
use the raw pointer.

BUG=610048

Review-Url: https://codereview.chromium.org/1963323004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393195}

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