[WebLayer] Add protected constructor to CookieManager for mocking

This matches what we do with various other classes like Profile to allow
the client to mock for tests.

Bug: 1054160
Change-Id: I9dc34dbf5309ea071aeeb1d0fd2081a171c82085
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2133877
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755846}

weblayer/public/java/org/chromium/weblayer/CookieManager.java

@@ -34,7 +34,12 @@ public class CookieManager {
         }
     }
 
-    CookieManager(ICookieManager impl) {
+    // Constructor for test mocking.
+    protected CookieManager() {
+        mImpl = null;
+    }
+
+    private CookieManager(ICookieManager impl) {
         mImpl = impl;
     }