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

tools/telemetry/telemetry/core/platform/desktop_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/desktop_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/desktop_platform_backend.py b/tools/telemetry/telemetry/core/platform/desktop_platform_backend.py
new file mode 100644
index 0000000000000000000000000000000000000000..103aff65510155e1b7316d150b3ea0623db86956
--- /dev/null
+++ b/tools/telemetry/telemetry/core/platform/desktop_platform_backend.py
@@ -0,0 +1,51 @@
+# Copyright (c) 2013 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import os
+import subprocess
+
+from telemetry.core import util
+from telemetry.core.platform import platform_backend
+
+
+class DesktopPlatformBackend(platform_backend.PlatformBackend):
+
+  # This is an abstract class. It is OK to have abstract methods.
+  # pylint: disable=W0223
+
+  def GetFlushUtilityName(self):
+    return NotImplementedError()
+
+  def _FindNewestFlushUtility(self):
+    flush_command = None
+    flush_command_mtime = 0
+
+    chrome_root = util.GetChromiumSrcDir()
+    for build_type, build_dir in util.BuildDirectoryIterator():
+      candidate = os.path.join(chrome_root, build_dir, build_type,
+                               self.GetFlushUtilityName())
+      if os.access(candidate, os.X_OK):
+        candidate_mtime = os.stat(candidate).st_mtime
+        if candidate_mtime > flush_command_mtime:
+          flush_command = candidate
+          flush_command_mtime = candidate_mtime
+
+    return flush_command
+
+  def FlushSystemCacheForDirectory(self, directory, ignoring=None):
+    assert directory and os.path.exists(directory), \
+        'Target directory %s must exist' % directory
+    flush_command = self._FindNewestFlushUtility()
+    assert flush_command, \
+        'You must build %s first' % self.GetFlushUtilityName()
+
+    args = [flush_command, '--recurse']
+    directory_contents = os.listdir(directory)
+    for item in directory_contents:
+      if item not in ignoring:
+        args.append(os.path.join(directory, item))
+
+    p = subprocess.Popen(args)
+    p.wait()
+    assert p.returncode == 0, 'Failed to flush system cache'