Don't force layout for mouse event hit tests

LayoutTests/http/tests/local/blob/script-tests/send-hybrid-blob.js

 description("Test for building blobs with multiple files combined by BlobBuilder and sending them via XMLHttpRequest.  (This test requires eventSender.beginDragWithFiles)");
 
 var testFilePaths = [
     "nonexistent",
     "empty.txt",
     "file-for-drag-to-send.txt",
     "file-for-drag-to-send2.txt",
     "abe.png"
 ];
 
@@ skipping 70 matching lines @@
                         F('file-for-drag-to-send.txt'),
                         F('file-for-drag-to-send2.txt'),
                         F('abe.png') ],
                       { 'start': 20, 'length': 3000 });
 
     testRunner.notifyDone();
 }
 
 if (window.eventSender) {
     testRunner.waitUntilDone();
+    // Mouse events only work after an initial layout
+    document.body.offsetLeft;
     util.runTests();
 } else
     testFailed("This test is not interactive, please run using DumpRenderTree");
 
 var successfullyParsed = true;