[Telemetry] Small fixes to power monitoring

tools/telemetry/telemetry/core/platform/mac_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/mac_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/mac_platform_backend.py b/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
index 89fe7f3923ceb48adc000a7032ace6e01c37243d..b391e221bf68f33b9f02bbd13635da3e7915a9bc 100644
--- a/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
@@ -80,7 +80,8 @@ class MacPlatformBackend(posix_platform_backend.PosixPlatformBackend):
         assert returncode in [0, -15], (
             "powermetrics return code: %d" % returncode)
 
-        return open(self._output_filename, 'rb').read()

[tonyg, 2014/02/13 15:30:37]

Just for my knowledge, does this really leak? Or just delay collection of the fd
until gc runs?

I use this pattern a lot and like the brevity, but will avoid it if it really
leaks.

+        with open(self._output_filename, 'rb') as output_file:
+          return output_file.read()
       finally:
         shutil.rmtree(self._ouput_directory)
         self._ouput_directory = None