Push API: Avoid false positive DCHECK.

PushMessagingAppIdentifier::FindByAppId was DCHECKing when called from
PushMessagingServiceImpl::CanHandle to see if an app_id belongs to
Push, since it assumes that it would only be called with valid Push
app_ids.

This patch makes it test whether the app_id is a Push Messaging app_id
before performing any validation DCHECKs.

BUG=489304

Review URL: https://codereview.chromium.org/1140873005

Cr-Commit-Position: refs/heads/master@{#330378}

chrome/browser/push_messaging/push_messaging_app_identifier.cc

@@ -75,9 +75,15 @@ PushMessagingAppIdentifier PushMessagingAppIdentifier::Generate(
 // static
 PushMessagingAppIdentifier PushMessagingAppIdentifier::FindByAppId(
     Profile* profile, const std::string& app_id) {
-  // Check case of app_id hasn't been mangled (crbug.com/461867).
-  DCHECK_GE(app_id.size(), kPrefixLength + kGuidLength);
+  if (!StartsWithASCII(app_id, kPushMessagingAppIdentifierPrefix,
+                       false /* case_sensitive */)) {
+    return PushMessagingAppIdentifier();
+  }
+
+  // Since we now know this is a Push Messaging app_id, check the case hasn't
+  // been mangled (crbug.com/461867).
   DCHECK_EQ(kPushMessagingAppIdentifierPrefix, app_id.substr(0, kPrefixLength));
+  DCHECK_GE(app_id.size(), kPrefixLength + kGuidLength);
   DCHECK_EQ(app_id.substr(app_id.size() - kGuidLength),
             StringToUpperASCII(app_id.substr(app_id.size() - kGuidLength)));
 

chrome/browser/push_messaging/push_messaging_app_identifier_unittest.cc

@@ -76,6 +76,13 @@ TEST_F(PushMessagingAppIdentifierTest, UniqueGuids) {
                 GURL("https://www.example.com/"), 1).app_id());
 }
 
+TEST_F(PushMessagingAppIdentifierTest, FindInvalidAppId) {
+  // These calls to FindByAppId should not DCHECK.
+  EXPECT_TRUE(PushMessagingAppIdentifier::FindByAppId(profile(), "").is_null());
+  EXPECT_TRUE(PushMessagingAppIdentifier::FindByAppId(
+      profile(), "amhfneadkjmnlefnpidcijoldiibcdnd").is_null());
+}
+
 TEST_F(PushMessagingAppIdentifierTest, PersistAndFind) {
   ASSERT_TRUE(PushMessagingAppIdentifier::FindByAppId(
       profile(), original_.app_id()).is_null());