Remove unnecessary ResourceRequest creation in synthetic response

This optimizes `ServiceWorkerSyntheticResponseManager::StartRequest()`
by removing an unnecessary copy of the `network::ResourceRequest`.

Previously, a new `ResourceRequest` was created to disable client-side
content decoding for synthetic responses. This change modifies the
`StartRequest` method not to create a new `ResourceRequest` just for
updating `client_side_content_decoding_enabled`. Instead, moves the
responsibility to the caller side. This avoids the overhead of creating
and copying the request object, improving performance.

Bug: 447039330
Change-Id: Ia842d5ecbd0357bcb6d9e66b4347bddb8741ca64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6978527
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520339}