Reland of "Remove BackForwardCacheImpl::UnloadSupportStrategy::kNo"

This CL relands the following CL with small difference.

[difference]
 - "unload_support = no" became the new default value for desktop.
 - Reintroduce kUnloadHandlerExistsInMainFrame (which is deprecated in
   https://crrev.com/c/2972031), and use it in this CL.

Currently, BFCache-Opt-In header is not fully standardized. So we
shouldn’t make the BFCache-Opt-In header default.

=====
Remove BackForwardCacheImpl::UnloadSupportStrategy::kNo

Bug: 1201653
Change-Id: Ifd4afb86776cabf2c8299ac4b75b493ea06f5aae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2951981
Auto-Submit: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#891079}
=====

Bug: 1201653
Change-Id: Ibe0b3a28d11f0e0dd2c14bd135b49d892529c1eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3127964
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917674}
NOKEYCHECK=True
GitOrigin-RevId: 75b51fd6800b4e4c17fff2b507416b17e48e41f3

blink/public/devtools_protocol/browser_protocol.pdl

@@ -7849,6 +7849,7 @@ domain Page
       BrowsingInstanceNotSwapped
       BackForwardCacheDisabledForDelegate
       OptInUnloadHeaderNotPresent
+      UnloadHandlerExistsInMainFrame
       UnloadHandlerExistsInSubFrame
       ServiceWorkerUnregistration
       CacheControlNoStore