Add feature flag: EmojiHandwritingVoiceInput to enable emoji, handwriting and voice input on opt-in IME menu.

The flag is available in chrome://flags page: #EmojiHandwritingVoiceInput
To run OS from command line: --enable-features=EmojiHandwritingVoiceInput

BUG=642385
TEST=Verified on local build

Review-Url: https://codereview.chromium.org/2306493002
Cr-Commit-Position: refs/heads/master@{#416543}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 620b688..2187b78 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2059,6 +2059,11 @@
      IDS_FLAGS_EXPERIMENTAL_POINTER_EVENT_V1_SPEC_CAPTURING_NAME,
      IDS_FLAGS_EXPERIMENTAL_POINTER_EVENT_V1_SPEC_CAPTURING_DESCRIPTION, kOsAll,
      FEATURE_VALUE_TYPE(features::kPointerEventV1SpecCapturing)},
+#if defined(OS_CHROMEOS)
+    {"enable-emoji-handwriting-voice-on-ime-menu",
+     IDS_FLAGS_ENABLE_EHV_INPUT_NAME, IDS_FLAGS_ENABLE_EHV_INPUT_DESCRIPTION,
+     kOsCrOS, FEATURE_VALUE_TYPE(features::kEHVInputOnImeMenu)},
+#endif
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in
     // histograms.xml and don't forget to run AboutFlagsHistogramTest unit test.