Add JSTypedArray's length in post-mortem metadata.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1363683002

Cr-Commit-Position: refs/heads/master@{#30873}

tools/gen-postmortem-metadata.py

@@ -150,6 +150,7 @@ extras_accessors = [
     'HeapObject, map, Map, kMapOffset',
     'JSObject, elements, Object, kElementsOffset',
     'FixedArray, data, uintptr_t, kHeaderSize',
+    'JSTypedArray, length, Object, kLengthOffset',
     'Map, instance_attributes, int, kInstanceAttributesOffset',
     'Map, inobject_properties_of_constructor_function_index_offset, int, kInObjectPropertiesOrConstructorFunctionIndexOffset',
     'Map, instance_size, int, kInstanceSizeOffset',