Add another issue to the known crashes.

BUG=21295

TEST=None

Review URL: http://codereview.chromium.org/200044

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@25670 0039d316-1c4b-4281-b951-d872f2087c98

chrome/test/data/reliability/known_crashes.txt

@@ -143,3 +143,6 @@ PREFIX : messageloop::dowork
 PREFIX : v8::internal::invoke
 PREFIX : webcore::renderlayer
 PREFIX : webcore::renderobject
+
+# 21295
+PREFIX : renderwidgethost::getbackingstore