Protect ENABLE_PROFILING to fix profiling=1.
Protect ENABLE_PROFILING to fix profiling=1.

Chromium defines ENABLE_PROFILING under the gyp flag profiling=1. This
corrects the resulting mulitple defintion error:
../../talk/base/profiler.h:61:9: error: 'ENABLE_PROFILING' macro redefined [-Werror]
#define ENABLE_PROFILING

and allows us to use profiling=1 in standalone builds.

TESTED=build passes with profiling=1
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/10829004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a5586b50e5cc19c0721a5d62ba8e950d16f637f9

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