PDF: Remove comment about flaky test which has been fixed

BUG=550015

Review URL: https://codereview.chromium.org/1471043003

Cr-Commit-Position: refs/heads/master@{#361278}

chrome/browser/pdf/pdf_extension_test.cc

@@ -542,7 +542,6 @@ IN_PROC_BROWSER_TEST_F(MaterialPDFExtensionTest, Elements) {
   RunTestsInFile("material_elements_test.js", "test.pdf");
 }
 
-// TODO(tsergeant): Verify that flake is fixed. http://crbug.com/550015
 IN_PROC_BROWSER_TEST_F(MaterialPDFExtensionTest, ToolbarManager) {
   RunTestsInFile("toolbar_manager_test.js", "test.pdf");
 }