[Perf] Fix browse:social:pinterest_infinite_scroll on mobile

Changes the login-complete detection code to search for the results grid
div rather than the search button which appears a little too early for
the login to complete.

Bug: 1160707
Change-Id: I55e321a61619ef9172ad6e196ca89d056b94ec91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2720124
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#857802}

tools/perf/page_sets/login_helpers/pinterest_login.py

@@ -60,5 +60,4 @@ def LoginMobileAccount(action_runner, credential,
     for a detailed list of possible exceptions.
   """
   _LoginAccount(action_runner, credential, credentials_path)
-  action_runner.WaitForElement(selector='svg[aria-label="Search"]')
-
+  action_runner.WaitForElement(selector='div.mobileGrid div[role="button"]')