Bug 1335880 - Adjust mouse position for dragstart. r=masayuki

On Ubuntu 16.04 TLS, test_dragdrop.html due to control margin.  So we should adjust the point of dragstart.

MozReview-Commit-ID: EPW2UnaIWRt

--HG--
extra : rebase_source : a97c66dc36328563497130aa7a3829bc8a9eb204

editor/libeditor/tests/test_dragdrop.html

@@ -48,7 +48,7 @@ function doTest()
 
   // -------- Test dragging text from an <input>
   input.setSelectionRange(1, 4);
-  result = synthesizeDragStart(input, [[{type: "text/plain", data: "rag"}]], window, 25, 6);
+  result = synthesizeDragStart(input, [[{type: "text/plain", data: "rag"}]], window, 25, 8);
   is(result, null, "Test dragging input");
 
   // -------- Test dragging text from a <textarea>