Adjust MD_CONTEXT_CPU_MASK to reflect reality

The value of MD_CONTEXT_CPU_MASK in use assumes that only the lower 6 bits are
used for flags, and the upper 26 bits are for the CPU type. However, as of
Windows 7 SP1, the 7th bit is being used as a flag (per
http://msdn.microsoft.com/en-us/library/hh134238%28v=vs.85%29.aspx and the
Windows SDK headers). Adjusting MD_CONTEXT_CPU_MASK works, but unfortunately
that masks off the existing value of MD_CONTEXT_ARM. This patch also changes the
value of MD_CONTEXT_ARM and adjusts the minidump context reading machinery to
gracefully handle minidumps with the old value.

[Ted Mielczarek]

Forgot to check "send mail" as usual...

[Mark Mentovai]

LGTM

http://breakpad.appspot.com/302001/diff/1/6
File src/processor/minidump.cc (right):

http://breakpad.appspot.com/302001/diff/1/6#newcode419
Line 419: context_flags |= MD_CONTEXT_ARM;
And unset MD_CONTEXT_ARM_OLD?