Add additional DVLOG output for origin trial token parsing

There is already some debug output available for origin trials,
which can be enabled with:

 % chrome --vmodule=origin_trial_context=1 --enable-logging=stderr

This CL adds a bit more human-readable debug output, plus it adds
the appropriate flags to the documentation, so it is easier to find.

TL/DR now:
 % chrome --vmodule=trial_token=2,origin_trial_context=1 --enable-logging=stderr

Change-Id: I27d08055f460287e02531886593c696e700bbe3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347385Reviewed-by: Jason Chase <chasej@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799308}

docs/origin_trials_integration.md

@@ -204,6 +204,17 @@ It's also used by Origin Trials unit tests and web tests.
 If you cannot set command-line switches (e.g., on Chrome OS), you can also
 directly modify [chrome_origin_trial_policy.cc].
 
+To see additional information about origin trial token parsing (including reasons
+for failures, or token names for successful tokens), you can add these switches:
+
+  `--vmodule=trial_token=2,origin_trial_context=1`
+
+If you are building with `is_debug=false`, then you will also need to add
+`dcheck_always_on=true` to your build options, and add this to the command line:
+
+  `--enable-logging=stderr`
+
+
 ### Web Tests
 When using the \[RuntimeEnabled\] IDL attribute, you should add web tests
 to verify that the V8 bindings code is working as expected. Depending on how

third_party/blink/common/origin_trials/trial_token.cc

@@ -7,6 +7,7 @@
 #include "base/base64.h"
 #include "base/big_endian.h"
 #include "base/json/json_reader.h"
+#include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/optional.h"
@@ -74,6 +75,7 @@ std::unique_ptr<TrialToken> TrialToken::From(
   *out_status = Extract(token_text, public_key, &token_payload,
                         &token_signature, &token_version);
   if (*out_status != OriginTrialTokenStatus::kSuccess) {
+    DVLOG(2) << "Malformed origin trial token found (unable to extract)";
     return nullptr;
   }
   std::unique_ptr<TrialToken> token = Parse(token_payload, token_version);
@@ -81,8 +83,11 @@ std::unique_ptr<TrialToken> TrialToken::From(
     token->signature_ = token_signature;
     *out_status = OriginTrialTokenStatus::kSuccess;
   } else {
+    DVLOG(2) << "Malformed origin trial token found (unable to parse)";
     *out_status = OriginTrialTokenStatus::kMalformed;
   }
+  DVLOG(2) << "Valid origin trial token found for feature "
+           << token->feature_name();
   return token;
 }
 
@@ -91,9 +96,11 @@ OriginTrialTokenStatus TrialToken::IsValid(const url::Origin& origin,
   // The order of these checks is intentional. For example, will only report a
   // token as expired if it is valid for the origin.
   if (!ValidateOrigin(origin)) {
+    DVLOG(2) << "Origin trial token from different origin";
     return OriginTrialTokenStatus::kWrongOrigin;
   }
   if (!ValidateDate(now)) {
+    DVLOG(2) << "Origin trial token expired";
     return OriginTrialTokenStatus::kExpired;
   }
   return OriginTrialTokenStatus::kSuccess;