[Telemetry] Add the ability to flush the system file cache to platform.

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 e415350f929d190c2da851124f761b237ce70750..2465cfcc05341952dd4ce6438af37b94351c29d6 100644
--- a/tools/telemetry/telemetry/core/platform/platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/platform_backend.py
@@ -48,3 +48,12 @@ class PlatformBackend(object):
 
   def GetOSVersionName(self):
     return None
+
+  def CanFlushIndividualFilesFromSystemCache(self):
+    raise NotImplementedError()
+
+  def FlushEntireSystemCache(self):
+    raise NotImplementedError()
+
+  def FlushSystemCacheForDirectory(self, directory, ignoring=None):
+    raise NotImplementedError()