[chrome-remote-control] Fix flakiness on docs.

We need to wait for the scrollable region to exist before
scrollable_element_function runs.

Example failure:
http://build.chromium.org/p/chromium.perf/builders/Linux%20Perf%20%282%29/builds/18255/steps/scrolling_benchmark/logs/stdio

BUG=None
TEST=Manual

Review URL: https://codereview.chromium.org/11226048

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@163436 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/page_sets/top_25.json

@@ -24,13 +24,15 @@
             "url": "https://drive.google.com",
             "why": "productivity, top google properties",
             "credentials": "google",
+            "wait_for_javascript_expression": "document.getElementsByClassName('doclistview-list').length",
             "scrollable_element_function": "function(callback) { callback(document.getElementsByClassName('doclistview-list')[0]); }"
         },
         {
             "name": "Docs  (1 open document tab)",
             "url": "https://docs.google.com/document/d/1X-IKNjtEnx-WW5JIKRLsyhz5sbsat3mfTpAPUSX3_s4/view",
             "why": "productivity, top google properties; Sample doc in the link",
             "credentials": "google",
+            "wait_for_javascript_expression": "document.getElementsByClassName('kix-appview-editor').length",
             "scrollable_element_function": "function(callback) { callback(document.getElementsByClassName('kix-appview-editor')[0]); }"
         },
         {