Adds function for activating a tab.

tools/telemetry/telemetry/browser_backend.py

Index: tools/telemetry/telemetry/browser_backend.py
diff --git a/tools/telemetry/telemetry/browser_backend.py b/tools/telemetry/telemetry/browser_backend.py
index a20c06aa7075d2d5f0e4568ac11f6ae029bd8db0..358cf6358c5dfdd6e6b60d4a34eb779d10a7ad70 100644
--- a/tools/telemetry/telemetry/browser_backend.py
+++ b/tools/telemetry/telemetry/browser_backend.py
@@ -60,6 +60,16 @@ class TabController(object):
     util.WaitFor(lambda: not self._FindTabInfo(debugger_url), timeout=5)
     self._UpdateTabList()
 
+  def ActivateTab(self, debugger_url, timeout=None):
+    assert debugger_url in self._tab_dict
+    tab_id = debugger_url.split('/')[-1]
+    try:
+      response = self._browser_backend.Request('activate/%s' % tab_id,
+                                               timeout=timeout)
+    except urllib2.HTTPError:
+      raise Exception('Unable to activate tab, tab id not found: %s' % tab_id)
+    assert response == 'Target activated'
+
   def GetTabUrl(self, debugger_url):
     tab_info = self._FindTabInfo(debugger_url)
     # TODO(hartmanng): crbug.com/166886 (uncomment the following assert and