Fix log error with GURL migration for PartnerBookmarks

Noticed in a logcat:
08-16 21:34:50.594118 16205 16226 W cr_PartnerCustomize: Partner homepage must be HTTP(S) or NewTabPage. Got invalid URL "org.chromium.url.GURL@0"

Change-Id: Ied8003b58e5f2585e32fb38f90748deac7a9fc2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3842454
Auto-Submit: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Peilin Wang <peilinwang@google.com>
Reviewed-by: Peilin Wang <peilinwang@google.com>
Cr-Commit-Position: refs/heads/main@{#1037802}

chrome/browser/partnercustomizations/java/src/org/chromium/chrome/browser/partnercustomizations/PartnerBrowserCustomizations.java

@@ -369,7 +369,7 @@ static boolean isValidHomepage(GURL url) {
             Log.w(TAG,
                     "Partner homepage must be HTTP(S) or NewTabPage. "
                             + "Got invalid URL \"%s\"",
-                    url);
+                    url.getPossiblyInvalidSpec());
             return false;
         }
         if (url.getSpec().length() > HOMEPAGE_URL_MAX_LENGTH) {