change file:// to asset:// to indicate that they aren't arbitrary files

refs #579

platform/default/application_root.cpp

@@ -0,0 +1,14 @@
+#include <mbgl/platform/platform.hpp>
+
+#include <mbgl/util/uv.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default cache database on this system.
+std::string applicationRoot() {
+    return uv::cwd();
+}
+
+}
+}