NaCl: Update revision in DEPS, r12952 -> r12975

NaCl: Update revision in DEPS, r12952 -> r12975

This pulls in the following Native Client changes:

r12953: (mseaborn) Unsandboxed IRT: Implement getpid()
r12954: (noelallen) Fix irt_syscalls.h dependency
r12955: (dyen) Toolchains no longer download tar balls unnecessarily.
r12956: (mseaborn) PNaCl driver scripts: Remove unused USE_BOOTSTRAP field
r12957: (mseaborn) IRT interfaces: Clean up use of off_t by defining a nacl_irt_off_t type
r12958: (noelallen) Fix roll
r12959: (noelallen) Roll bionic toolchain.
r12960: (noelallen) Update to builder output
r12961: (mseaborn) PNaCl driver scripts: Fix various pyflakes warnings
r12962: (dyen) Package archives now have their own json description files.
r12963: (mseaborn) Fix #including of <irt_dev.h>, since nacl_io requires this to work
r12964: (jvoung) Test that gdb tests work with the sandboxed translator.
r12965: (mseaborn) PNaCl driver scripts: Remove use of "import *"
r12966: (mseaborn) Unsandboxed IRT: Whitelist and convert mmap() flag values
r12967: (bradnelson) Provide chrome the option of learning the debug stub port chosen by nacl.
r12968: (dyen) Toolchain Build now stores package description files for final packages.
r12969: (jvoung) PNaCl: Update LLVM revision in pnacl/COMPONENT_REVISIONS
r12971: (mseaborn) pnacl-translate: Refactor selection of arch-specific flags
r12972: (jvoung) Clean up spec2k scripts a bit.
r12973: (dyen) buildbot scripts changes should trigger all trybots.
r12974: (bradnelson) Adding environment variable on the bots to look for msvs2013.

BUG=none
TEST=browser_tests and nacl_integration