[EoC] Add a feature flag for the opt-out

Add a feature flag that controls whether opting out of EoC is enabled.
The opt-out is enabled by default so that users in our live experiment
groups don't lose their ability to opt-out. When experimentation
configurations are updated, we will have the ability to disable the
opt-out for new groups.

Also fixes a bug where the toolbar button incorrectly started showing up
on all tabs after opting out.

BUG=859564

Change-Id: Ib0e5f90a0f3c80a928688ddd8bb33036b7b5e247
Reviewed-on: https://chromium-review.googlesource.com/1123098
Reviewed-by: Becky Zhou <[email protected]>
Reviewed-by: Filip Gorski <[email protected]>
Commit-Queue: Theresa <[email protected]>
Cr-Commit-Position: refs/heads/master@{#572003}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 3b8ee38..0df2628 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2536,6 +2536,10 @@
      flag_descriptions::kContextualSuggestionsSlimPeekUIDescription, kOsAndroid,
      FEATURE_VALUE_TYPE(
          contextual_suggestions::kContextualSuggestionsSlimPeekUI)},
+    {"contextual-suggestions-opt-out",
+     flag_descriptions::kContextualSuggestionsOptOutName,
+     flag_descriptions::kContextualSuggestionsOptOutDescription, kOsAndroid,
+     FEATURE_VALUE_TYPE(contextual_suggestions::kContextualSuggestionsOptOut)},
     {"enable-content-suggestions-new-favicon-server",
      flag_descriptions::kEnableContentSuggestionsNewFaviconServerName,
      flag_descriptions::kEnableContentSuggestionsNewFaviconServerDescription,