Add a new chrome flag for Ambient Mode.

This CL adds a new flag in chrome://flags to enable/disable the Ambient
Mode feature.

Bug: b/149244227
Test: manually.
Change-Id: I7f07020db81fc61962bbe923c62ce1837634d94c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053037
Commit-Queue: Meilin Wang <[email protected]>
Reviewed-by: Tao Wu <[email protected]>
Reviewed-by: Xiaohui Chen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#742284}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 4f1d62c..ab875ce 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -4975,6 +4975,12 @@
      flag_descriptions::kExportTaggedPDFDescription, kOsDesktop,
      FEATURE_VALUE_TYPE(features::kExportTaggedPDF)},
 
+#if defined(OS_CHROMEOS)
+    {"enable-ambient-mode", flag_descriptions::kEnableAmbientModeName,
+     flag_descriptions::kEnableAmbientModeDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(chromeos::features::kAmbientModeFeature)},
+#endif  // defined(OS_CHROMEOS)
+
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag
     // Histograms" in tools/metrics/histograms/README.md (run the