Breakpad processor: Work around overload resolution problems in stream pos_type comparisons

When building with G++ 4.1.2, src/processor/cfi_frame_info.cc fails to
build with the error below. G++ 4.2.1 and later do not seem to report this
problem.

This patch works around the problem by casting stream.tellp() to
std::streamoff before doing the comparison.

src/processor/cfi_frame_info.cc: In member function `std::string
google_breakpad::CFIFrameInfo::Serialize() const':
src/processor/cfi_frame_info.cc:105: error: ambiguous overload for `operator!='
in `stream.std::basic_ostringstream<char, std::char_traits<char>,
std::allocator<char> >::<anonymous>.std::basic_ostream<_CharT, _Traits>::tellp
[with _CharT = char, _Traits = std::char_traits<char>]() != 0'
src/processor/cfi_frame_info.cc:105: note: candidates are:
operator!=(std::streamoff, int) <built-in>
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/postypes.h:143:
note:                 bool std::fpos<_StateT>::operator!=(const
std::fpos<_StateT>&) const [with _StateT = __mbstate_t]

[jimb]

Here's a patch to work around a build problem with G++ 4.1.2.  It's innocent on
4.2.1 and 4.4.1.

[jimb]

LGTM'd by mail.  Committed, r572.