dump_context: arm: name the registers in the dump output
dump_context: arm: name the registers in the dump output

The current ARM minidump_dump output makes people remember or look up
how registers are mapped in the ISA.  Let's use human friendly names
instead so they don't have to.

Currently it looks like:
MDRawContextARM
  context_flags       = 0x40000006
  iregs[ 0]            = 0x3c48b000
  iregs[ 1]            = 0x3
  iregs[ 2]            = 0x20
  iregs[ 3]            = 0x0
  iregs[ 4]            = 0x1c
  iregs[ 5]            = 0x3c48b000
  iregs[ 6]            = 0x20
  iregs[ 7]            = 0x3c48b04c
  iregs[ 8]            = 0x39100611
  iregs[ 9]            = 0x1c
  iregs[10]            = 0x0
  iregs[11]            = 0xbe61c200
  iregs[12]            = 0xfb9c1fec
  iregs[13]            = 0xbe61bd28
  iregs[14]            = 0x39e19b1c
  iregs[15]            = 0x357dd74c
  cpsr                = 0x680b0010
  float_save.fpscr     = 0x0

Now it looks like:
MDRawContextARM
  context_flags        = 0x40000006
  r0                   = 0x3c48b000
  r1                   = 0x3
  r2                   = 0x20
  r3                   = 0x0
  r4                   = 0x1c
  r5                   = 0x3c48b000
  r6                   = 0x20
  r7                   = 0x3c48b04c
  r8                   = 0x39100611
  r9                   = 0x1c
  r10                  = 0x0
  r11                  = 0xbe61c200
  r12                  = 0xfb9c1fec
  sp                   = 0xbe61bd28
  lr                   = 0x39e19b1c
  pc                   = 0x357dd74c
  cpsr                 = 0x680b0010
  float_save.fpscr     = 0x0

BUG=chromium:665083

Change-Id: I46d87c4ff7303a7efcd60da1d0b67ae7a5465c8f
Reviewed-on: https://chromium-review.googlesource.com/457197
Reviewed-by: Ivan Penkov <46ceba68fbb08a68af41e6bf9f65df3635ba035c@chromium.org>

bittorrent/breakpad,bittorrent/breakpad,bittorrent/breakpad,bittorrent/breakpad,bittorrent/breakpad,bittorrent/breakpad,bittorrent/breakpad