Navigation: Remove unused NavigationState::set_navigation_start()

This CL also removes the outdated comments on
mojom::NavigationParams::navigation_start.

Bug: n/a
Change-Id: I4672611f52d861b797f2a472496813da6de35875
Tbr: kinuko@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132069
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Hemery <ahemery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758771}

content/common/navigation_params.mojom

@@ -200,10 +200,6 @@ struct CommonNavigationParams {
   int32 previews_state;
 
   // The navigationStart time exposed through the Navigation Timing API to JS.
-  // If this is for a browser-initiated navigation, this can override the
-  // navigation_start value in Blink.
-  // For renderer initiated navigations, this will be set on the
-  // renderer side and sent with FrameHost::BeginNavigation.
   mojo_base.mojom.TimeTicks navigation_start;
 
   // The request method: GET, POST, etc.