Make telemetry platform a singleton

tools/telemetry/telemetry/core/possible_browser.py

Index: tools/telemetry/telemetry/core/possible_browser.py
diff --git a/tools/telemetry/telemetry/core/possible_browser.py b/tools/telemetry/telemetry/core/possible_browser.py
index cd1403327033862eddfc65460fcbd3855b2296e2..0f34b0e560b1acba05ec4ca298dedb7876e7cc5b 100644
--- a/tools/telemetry/telemetry/core/possible_browser.py
+++ b/tools/telemetry/telemetry/core/possible_browser.py
@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-from telemetry.core import platform
-
 
 class PossibleBrowser(object):
   """A browser that can be controlled.
@@ -41,10 +39,12 @@ class PossibleBrowser(object):
 
   @property
   def platform(self):
-    if not self._platform:
-      self._platform = platform.Platform(self._platform_backend)
+    self._InitPlatformIfNeeded()
     return self._platform
 
+  def _InitPlatformIfNeeded(self):
+    raise NotImplementedError()
+
   def Create(self):
     raise NotImplementedError()