Do not call internals.settings.setOverlayScrollbarsEnabled() in onload

internals.settings.setOverlayScrollbarsEnabled() doesn't work well when it is
called after ScrollbarTheme is initialized in the first layout.

The CL [1] modifies the timing of document load event and makes onload() in
fast/scrolling/scrollbar-tickmarks-hittest.html to be called after the first
layout, and thus breaks the test.

This CL moves setOverlayScrollbarsEnabled() to outside onload() such that it
is called before ScrollbarTheme is initialized.

[1] https://codereview.chromium.org/2269043002/

BUG=624697
TEST=fast/scrolling/scrollbar-tickmarks-hittest.html

Review-Url: https://codereview.chromium.org/2484613003
Cr-Commit-Position: refs/heads/master@{#430284}

third_party/WebKit/LayoutTests/fast/scrolling/scrollbar-tickmarks-hittest.html

@@ -5,6 +5,10 @@
 
 <script>
 description('Checks that clicking on scrollbar works when tickmarks are added.');
+if (window.internals) {
+    window.internals.settings.setOverlayScrollbarsEnabled(true);
+}
+
 onload = function()
 {
     if (window.testRunner)
@@ -13,7 +17,6 @@ onload = function()
     var elt = document.getElementById('elt');
     range.selectNodeContents(elt);
     if (window.internals) {
-        window.internals.settings.setOverlayScrollbarsEnabled(true);
         window.internals.addTextMatchMarker(range, true);
     }
     if (window.eventSender) {