[New Multicolumn] Make computeOffsetFromCompositedAncestor() flowthread-aware.

LayoutTests/VirtualTestSuites

 [
   {
     "prefix": "gpu",
     "base": "fast/canvas",
     "args": ["--enable-accelerated-2d-canvas",
              "--disable-display-list-2d-canvas"]
   },
   {
     "prefix": "gpu",
     "base": "canvas/philip",
@@ skipping 103 matching lines @@
     "args": ["--enable-direct-write",
              "--enable-font-antialiasing"]
   },
   {
     "prefix": "threaded",
     "base": "printing",
     "args": ["--enable-threaded-compositing"]
   },
   {
     "prefix": "regionbasedmulticol",
+    "base": "compositing/columns",
+    "args": ["--enable-region-based-columns"]
+  },
+  {
+    "prefix": "regionbasedmulticol",
     "base": "fast/multicol",
     "args": ["--enable-region-based-columns", "--enable-impl-side-painting"]
   },
   {
     "prefix": "regionbasedmulticol",
     "base": "fast/pagination",
     "args": ["--enable-region-based-columns", "--enable-impl-side-painting"]
   },
   {
     "prefix": "slimmingpaint",
@@ skipping 34 matching lines @@
     "prefix": "slimmingpaint",
     "base": "fast/borders",
     "args": ["--enable-slimming-paint", "--enable-impl-side-painting"]
   },
   {
     "prefix": "rootlayerscrolls",
     "base": "fast/scrolling",
     "args": ["--root-layer-scrolls"]
   }
 ]