[typedarray] Move JSArrayBuffer functionality to js-array-buffer.cc

This is a cleanup CL that is preparation for making the byte length
and offsets of typed arrays into unboxed size_t fields.

R=mstarzinger@chromium.org
BUG=v8:7881

Change-Id: Iee8bb2142124c88c71cec7343149ae3f08b40c6c
Reviewed-on: https://chromium-review.googlesource.com/1179829Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55196}

BUILD.gn

@@ -2161,6 +2161,7 @@ v8_source_set("v8_base") {
     "src/objects/intl-objects.cc",
     "src/objects/intl-objects.h",
     "src/objects/js-array-buffer-inl.h",
+    "src/objects/js-array-buffer.cc",
     "src/objects/js-array-buffer.h",
     "src/objects/js-array-inl.h",
     "src/objects/js-array.h",