Trust Tokens: Add a flag for platform-provided trust token issuance

This change adds a flag option to enable platform-provided trust token
issuance (https://bit.ly/platform-provided-trust-tokens) in order to
make manual testing easier.

R=svaldez

Bug: 1130248
Test: Confirmed old "Enabled" option -> PPTT off, new option -> PPTT on
Change-Id: I191b08172147b3056a268fd630fcd07d7af6a5a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616140
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841589}

chrome/browser/about_flags.cc

@@ -2488,6 +2488,15 @@ const FeatureEntry::FeatureVariation
 };
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
 
+constexpr FeatureEntry::FeatureParam kPlatformProvidedTrustTokenIssuance[] = {
+    {"PlatformProvidedTrustTokenIssuance", "true"}};
+
+constexpr FeatureEntry::FeatureVariation
+    kPlatformProvidedTrustTokensVariations[] = {
+        {"with platform-provided trust token issuance",
+         kPlatformProvidedTrustTokenIssuance,
+         base::size(kPlatformProvidedTrustTokenIssuance), nullptr}};
+
 // RECORDING USER METRICS FOR FLAGS:
 // -----------------------------------------------------------------------------
 // The first line of the entry is the internal name.
@@ -6335,7 +6344,9 @@ const FeatureEntry kFeatureEntries[] = {
 
     {"trust-tokens", flag_descriptions::kTrustTokensName,
      flag_descriptions::kTrustTokensDescription, kOsAll,
-     FEATURE_VALUE_TYPE(network::features::kTrustTokens)},
+     FEATURE_WITH_PARAMS_VALUE_TYPE(network::features::kTrustTokens,
+                                    kPlatformProvidedTrustTokensVariations,
+                                    "TrustTokenOriginTrial")},
 
 #if defined(OS_ANDROID)
     {"android-partner-customization-phenotype",

services/network/public/cpp/features.cc

@@ -202,6 +202,9 @@ const base::FeatureParam<TrustTokenOriginTrialSpec>
 // Determines whether Trust Tokens issuance requests should be diverted, at the
 // corresponding issuers' request, to the operating system instead of sent
 // to the issuers' servers.
+//
+// WARNING: If you rename this param, you must update the corresponding flag
+// entry in about_flags.cc.
 const base::FeatureParam<bool> kPlatformProvidedTrustTokenIssuance{
     &kTrustTokens, "PlatformProvidedTrustTokenIssuance", false};