Fix error in layout trace viewer
auto merge of #4186 : mbrubeck/servo/layout-viewer, r=glennw

d1b433a3b3bab353f320b2f39fa953ce326d2d55 changed the encoding format for Flows.  The root is no longer wrapped inside a `data` field, but instead includes BaseFlow fields directly.  This broke the layout trace viewer, which threw an uncaught exception when the `data` property was missing.  This fixes the exception, though some functionality may still be partly broken.