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This is a follow-up CL for https://crrev.com/c/1623611.

|ServiceWorkerProviderHost::web_contents_getter_| can be created from
|frame_tree_node_id| in ctor. |web_contents_getter_| can be const.

This makes code a bit clean.

Bug: 623464
Change-Id: I8e8258a2e2579de2ef2e8e29a4da289d39b0346f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623664
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664661}