Don't expose FrameTreeNodeIdRegistry::Get()

This is a follow-up CL for https://crrev.com/c/1623611/7, addressing the comment [1].

[1] https://crrev.com/c/1623611/7/content/browser/frame_host/frame_tree_node_id_registry.h#31

Bug: 623464

Change-Id: I38bf8f5b0b22577bc278797151b20e9700700879
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628495
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663471}
4 files changed