No need to have an indirection to get to the initial JSArray maps from
the native context; we only cache the fast elements maps anyway, so
those could live on the native context directly. This will also
integrate nicely with the load/store propagation in TurboFan (once we
propagate the immutable flag for FieldAccess as well).

Drive-by-fix: Also don't embed any of the initial JSArray maps in
TurboFan generated code when allocating a new JSArray, but instead
always load the appropriate map from the native context.  This way
we ensure that we never leak a reference to one of those maps and
its as efficient as embedding a constant map.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1516433005

Cr-Commit-Position: refs/heads/master@{#32779}