DevTools protocol: take Fetch out of experimental

This has been stable for the past year and used by several clients already.

Change-Id: If4de63316c3da458a81d034d69aff29ae8889c28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410339Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806825}

third_party/blink/public/devtools_protocol/browser_protocol.pdl

@@ -7698,7 +7698,7 @@ experimental domain Tracing
       optional StreamCompression streamCompression
 
 # A domain for letting clients substitute browser's network layer with client code.
-experimental domain Fetch
+domain Fetch
   depends on Network
   depends on IO
   depends on Page
@@ -7709,12 +7709,12 @@ experimental domain Fetch
   # Stages of the request to handle. Request will intercept before the request is
   # sent. Response will intercept after the response is received (but before response
   # body is received.
-  experimental type RequestStage extends string
+  type RequestStage extends string
     enum
       Request
       Response
 
-  experimental type RequestPattern extends object
+  type RequestPattern extends object
     properties
       # Wildcards ('*' -> zero or more, '?' -> exactly one) are allowed. Escape character is
       # backslash. Omitting is equivalent to "*".
@@ -7731,7 +7731,7 @@ experimental domain Fetch
       string value
 
   # Authorization challenge for HTTP status code 401 or 407.
-  experimental type AuthChallenge extends object
+  type AuthChallenge extends object
     properties
       # Source of the authentication challenge.
       optional enum source
@@ -7745,7 +7745,7 @@ experimental domain Fetch
       string realm
 
   # Response to an AuthChallenge.
-  experimental type AuthChallengeResponse extends object
+  type AuthChallengeResponse extends object
     properties
       # The decision on what to do in response to the authorization challenge.  Default means
       # deferring to the default behavior of the net stack, which will likely either the Cancel