Add a finch flag to disable ContextualSearch selection expansion.

chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java

Index: chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java b/chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java
index 21c2a4eaa893d616a0d1583bb03a47d6ce4d034c..61a451b53f45ff4fa92e18d5892226885c58c0dd 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchFieldTrial.java
@@ -42,6 +42,9 @@ public class ContextualSearchFieldTrial {
     static final String SIDE_SEARCH_PROVIDER_ICON_ENABLED =
             "contextual_search_side_search_provider_icon_enabled";
 
+    static final String SELECTION_EXPANSION_DISABLED =
+            "contextual_search_selection_expansion_disabled";
+
     private static final String CHINESE_LANGUAGE_CODE = "zh";
     private static final String JAPANESE_LANGUAGE_CODE = "ja";
     private static final String KOREAN_LANGUAGE_CODE = "ko";
@@ -250,6 +253,13 @@ public class ContextualSearchFieldTrial {
         return getBooleanParam(SIDE_SEARCH_PROVIDER_ICON_ENABLED);
     }
 
+    /**
+     * @return Whether the base page selection expansion after server response is disabled.
+     */
+    public static boolean isSelectionExpansionDisabled() {
+        return getBooleanParam(SELECTION_EXPANSION_DISABLED);
+    }
+
     // --------------------------------------------------------------------------------------------
     // Helpers.
     // --------------------------------------------------------------------------------------------