RawPtr: fix boolean operations

Before the change RawPtr used to have following problems with boolean
operations:
1) Conversion from RawPtr to boolean was ambiguous as it had two
conversion operators to a pointer type at the same time:

'operator UnspecifiedBoolType() const' and 'operator T*() const'

2) 'operator UnspecifiedBoolType() const' caused a compilation error
when used, the reason: impossible conversion from 'T**' type to
'UnspecifiedBoolType' (to fix it '&RawPtr::m_ptr' should have been
returned instead of '&m_ptr')

3) Having of 'operator UnspecifiedBoolType() const' is only safe
when the == and != operators are declared explicitly, as otherwise
"RawPtr ==  RawPtr" could do wrong thing (i.e., convert to
UnspecifiedBoolType and then do the comparison). And comparison
operators were not declared explicitly.

Simple removal of 'operator UnspecifiedBoolType() const' and relying
on 'operator T*() const' for boolean operations fixes all problems
described above.

Review URL: https://codereview.chromium.org/149473003

git-svn-id: svn://svn.chromium.org/blink/trunk@166194 bbb929c8-8fbe-4397-9dbb-9b2b20218538