Make telemetry platform a singleton

tools/telemetry/telemetry/core/platform/platform_backend.py

Index: tools/telemetry/telemetry/core/platform/platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/platform_backend.py b/tools/telemetry/telemetry/core/platform/platform_backend.py
index e91148c7b9f2283cd3f3d1e45888c2162ee7638b..77e3ff7d82f7941028607300e49aae5383e7a128 100644
--- a/tools/telemetry/telemetry/core/platform/platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/platform_backend.py
@@ -37,6 +37,17 @@ MAVERICKS =    OSVersion('mavericks',    10.9)
 
 
 class PlatformBackend(object):
+  def __init__(self):
+    self._platform = None
+
+  def SetPlatform(self, platform):
+    assert self._platform == None
+    self._platform = platform
+
+  @property
+  def platform(self):
+    return self._platform
+
   def IsRawDisplayFrameRateSupported(self):
     return False