Split blink::binding::ObservableArrayExoticObjectImpl into its own file.

With ObservableArrayExoticObjectImpl in its own file, it can move
private static members into an anonymous namespace and not have to
expose them in the header. This in turn means one fewer include in
observable_array.h, which is included in 12% of the translation units
used to build the chrome target. The only thing in observable_array.cc
is the ObservableArrayExoticObjectImpl code anyway.

Change-Id: Idc045d01fab5ec263488778ea0eeb5e5f071f2c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3367470
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956163}