Add Dart class for _ExternalUint8ClampedArray and API to allocate it.

runtime/lib/byte_array.cc

Index: runtime/lib/byte_array.cc
===================================================================
--- runtime/lib/byte_array.cc	(revision 17039)
+++ runtime/lib/byte_array.cc	(working copy)
@@ -674,6 +674,18 @@
 }
 
 
+// ExternalUint8ClampedArray
+
+DEFINE_NATIVE_ENTRY(ExternalUint8ClampedArray_getIndexed, 2) {
+  UNALIGNED_GETTER(ExternalUint8ClampedArray, Smi, uint8_t);
+}
+
+
+DEFINE_NATIVE_ENTRY(ExternalUint8ClampedArray_setIndexed, 3) {
+  UNALIGNED_SETTER(ExternalUint8ClampedArray, Smi, Value, uint8_t);
+}
+
+
 // ExternalInt16Array
 
 DEFINE_NATIVE_ENTRY(ExternalInt16Array_getIndexed, 2) {