This cl does the following work:

- adds the flag of enabling opt-in IME menu in the about page.
- updates the string "IDS_OPTIONS_SETTINGS_LANGUAGES_ACTIVATE_IME_MENU" in the language page as what UI reviewer suggests.

BUG=570761
TEST=None

Review URL: https://codereview.chromium.org/1818863002

Cr-Commit-Position: refs/heads/master@{#383006}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 713c62d..320b7ec 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1822,6 +1822,9 @@
      IDS_FLAGS_EXPERIMENTAL_ACCESSIBILITY_FEATURES_DESCRIPTION, kOsCrOS,
      SINGLE_VALUE_TYPE(
          chromeos::switches::kEnableExperimentalAccessibilityFeatures)},
+     {"opt-in-ime-menu", IDS_FLAGS_ENABLE_IME_MENU_NAME,
+     IDS_FLAGS_ENABLE_IME_MENU_DESCRIPTION, kOsCrOS,
+     FEATURE_VALUE_TYPE(features::kOptInImeMenu)},
 #endif
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in