Add close method for Chrome

This adds a close method for the Chrome class and calls it during cleanup in
power_LoadTest. This makes power_LoadTest close Chrome when it finishes.

BUG=chromium:393027
TEST=run power_LoadTest

Change-Id: Ic3511a3c1908040a5b90e99645980b4bd35a9a68
Reviewed-on: https://chromium-review.googlesource.com/207688
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Derek Basehore <dbasehore@chromium.org>
Tested-by: Derek Basehore <dbasehore@chromium.org>
diff --git a/client/common_lib/cros/chrome.py b/client/common_lib/cros/chrome.py
index e63fc6c..0a7146c 100644
--- a/client/common_lib/cros/chrome.py
+++ b/client/common_lib/cros/chrome.py
@@ -98,7 +98,7 @@
 
 
     def __exit__(self, *args):
-        self.browser.Close()
+        self.close()
 
 
     @property
@@ -174,3 +174,8 @@
                 exceptions.BrowserConnectionGoneException):
             return True
         return False
+
+
+    def close(self):
+        """Closes the browser."""
+        self._browser.Close()