Import Microsoft's CONTEXT_ARM definitions

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Modern Windows SDKs have a CONTEXT_ARM. Thankfully the context cpu flag they're 
using doesn't conflict with any of the ones we currently have:
#define CONTEXT_ARM   0x00200000L

...but we should get that into the headers just so we don't create a conflict 
in the future. The context struct they use is about 90% compatible with the 
existing Breakpad struct, but I think the float state and debug registers they 
have after the GPRs doesn't match up. We could probably fiddle things to make 
our code handle either struct. (Breakpad doesn't use the float registers for 
anything, it'd just have to handle alternate-sized contexts.)

Original issue reported on code.google.com by ted.mielczarek on 2 Apr 2015 at 12:34