Refactor use of startsWith into indexOf === 0

The `String#startsWith` method is not supported in IE, but its behavior
can be replicated by testing whether the index of a string is equal to
zero. Replace instances of `startsWith` with such `indexOf` usages.

Change-Id: I37b5831900ff836545e224ac2f715b26ca694bbf

polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js

diff --git a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
index 32dd687..9c0e902 100644
--- a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
+++ b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
@@ -111,7 +111,7 @@
     locationChanged: function() {
       var page = '';
       var pathname = this._getPathname();
-      if (pathname.startsWith('/q/')) {
+      if (pathname.indexOf('/q/') === 0) {
         page = '/q/';
       } else if (pathname.match(CHANGE_VIEW_REGEX)) { // change view
         page = '/c/';

polygerrit-ui/app/elements/gr-app.js

diff --git a/polygerrit-ui/app/elements/gr-app.js b/polygerrit-ui/app/elements/gr-app.js
index 8ac268e..a188ea1 100644
--- a/polygerrit-ui/app/elements/gr-app.js
+++ b/polygerrit-ui/app/elements/gr-app.js
@@ -166,7 +166,7 @@
     _handleLocationChange: function(e) {
       var hash = e.detail.hash.substring(1);
       var pathname = e.detail.pathname;
-      if (pathname.startsWith('/c/') && parseInt(hash, 10) > 0) {
+      if (pathname.indexOf('/c/') === 0 && parseInt(hash, 10) > 0) {
         pathname += '@' + hash;
       }
       this.set('_path', pathname);