Allocate space in the backing store for computed property names.

The property backing store was pre-allocated for the constant
properties up to the first non-constant (computed name) property.
To use lowering for storing data properties in literals
with computed property names effectively, a fast store is needed, i.e.,
available space in the property backing store for properties
with computed names.

backing_store_size is the number of all properties (including
computed names, but without __proto__)
that is calculated in the ast and passed to the runtime function that allocates
the property backing store. backing_store_size and
constant_properties constitute a BoilerplateDescription.

backing_store_size might be slightly too high because computed names
can evaluate to the same name, but that should be a rare
case so over-allocating is OK.

If a property is __proto__, we don't store it as a regular
property, because the map changes. Keep track of
has_seen_proto in the parser to calculate the
backing store size correctly.

BUG=v8:5625

Review-Url: https://codereview.chromium.org/2632503003
Cr-Commit-Position: refs/heads/master@{#42576}