Restrict visibility of enable-emoji-context-menu flag to Mac.

Bug: 827404
Change-Id: I70bbde4b697f26069c79fce93af65a677889616f
Reviewed-on: https://chromium-review.googlesource.com/1036194
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554948}

chrome/browser/about_flags.cc

@@ -3820,10 +3820,12 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_VALUE_TYPE(features::kViewsCastDialog)},
 #endif  // defined(TOOLKIT_VIEWS)
 
+#if defined(OS_MACOSX)
     {"enable-emoji-context-menu",
      flag_descriptions::kEnableEmojiContextMenuName,
      flag_descriptions::kEnableEmojiContextMenuDescription, kOsDesktop,
      FEATURE_VALUE_TYPE(features::kEnableEmojiContextMenu)},
+#endif  // OS_MACOSX
 
     {"SupervisedUserCommittedInterstitials",
      flag_descriptions::kSupervisedUserCommittedInterstitialsName,