Expose flag to disable document mode in about:flags

Needed for testing on unrooted devices that can't set
the command line.

Requires flag caching gymnastics because the flag is used
before native has a chance to even load up the command
line flags.

BUG=582539

Review URL: https://codereview.chromium.org/1846723004

Cr-Commit-Position: refs/heads/master@{#384512}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 62a9d06..b853309c 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1692,6 +1692,10 @@
     {"tab-management-experiment-type", IDS_FLAGS_HERB_PROTOTYPE_CHOICES_NAME,
      IDS_FLAGS_HERB_PROTOTYPE_CHOICES_DESCRIPTION, kOsAndroid,
      MULTI_VALUE_TYPE(kHerbPrototypeChoices)},
+    {"enable-forced-migration-to-tabbed-mode",
+     IDS_FLAGS_FORCE_MIGRATION_TO_TABBED_MODE_NAME,
+     IDS_FLAGS_FORCE_MIGRATION_TO_TABBED_MODE_DESCRIPTION, kOsAndroid,
+     SINGLE_VALUE_TYPE(switches::kEnableForcedMigrationToTabbedMode)},
     {"enable-tab-switcher-in-document-mode",
      IDS_FLAGS_TAB_SWITCHER_IN_DOCUMENT_MODE_NAME,
      IDS_FLAGS_TAB_SWITCHER_IN_DOCUMENT_MODE_DESCRIPTION, kOsAndroid,