Updated url_constants.cc override.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/449db409a6840ea662c7fb65eb3c81f81a23f3af

commit 449db409a6840ea662c7fb65eb3c81f81a23f3af
Author: Joe DeBlasio <[email protected]>
Date:   Tue Jan 28 00:01:51 2020 +0000

    [MIX-DL] Add support for mixed content download warnings.

    This CL adds support for, but never triggers, two kinds of mixed content
    download warnings that show up in the download shelf. Neither is used
    yet. These supplement the existing silent blocking.

    The warnings look like this:
    - https://drive.google.com/file/d/1HKKrRv4uScGsVv_l8J9huZLnHWp-j_t6/view
    - https://drive.google.com/file/d/1TgSTEFh1n0gWn1CpJnqkP4imhtkk20gd/view

    This CL also adds support for these warnings in the chrome://downloads
    UI. There it re-uses the "dangerous" download warning without any
    modifications beyond a custom string.

    All MIX-DL blocking logic is guarded by a flag, but even when that flag
    is enabled, this change is not triggered and is thus not visible.
    Subsequent CLs will:
     - trigger these warnings when appropriate,
     - finish the UI for Android, and
     - add proper testing.

    This is part of go/downloads-as-mixed-content

    Bug: 960819

chromium_src/chrome/common/url_constants.cc

@@ -149,6 +149,9 @@ const char kLegacySupervisedUserManagementURL[] =
 
 const char kManagedUiLearnMoreUrl[] = "https://support.brave.com/";
 
+const char kMixedContentDownloadBlockingLearnMoreUrl[] =
+    "https://support.brave.com/";
+
 const char kMyActivityUrlInClearBrowsingData[] =
     "https://support.brave.com/";