Adding new string constants to remove usage of hardcoded string values.

BUG=None

Review URL: https://codereview.chromium.org/651853002

Cr-Commit-Position: refs/heads/master@{#299830}

chrome/android/java/src/org/chromium/chrome/browser/ChromeBrowserProvider.java

@@ -84,6 +84,14 @@ public class ChromeBrowserProvider extends ContentProvider {
         "vnd.android.cursor.dir/browser-history";
     private static final String BROWSER_CONTRACT_HISTORY_CONTENT_ITEM_TYPE =
         "vnd.android.cursor.item/browser-history";
+    private static final String BROWSER_CONTRACT_BOOKMARK_CONTENT_TYPE =
+        "vnd.android.cursor.dir/bookmark";
+    private static final String BROWSER_CONTRACT_BOOKMARK_CONTENT_ITEM_TYPE =
+        "vnd.android.cursor.item/bookmark";
+    private static final String BROWSER_CONTRACT_SEARCH_CONTENT_TYPE =
+        "vnd.android.cursor.dir/searches";
+    private static final String BROWSER_CONTRACT_SEARCH_CONTENT_ITEM_TYPE =
+        "vnd.android.cursor.item/searches";
 
     // This Authority is for internal interface. It's concatenated with
     // Context.getPackageName() so that we can install different channels
@@ -528,14 +536,14 @@ public class ChromeBrowserProvider extends ContentProvider {
         switch (match) {
             case URI_MATCH_BOOKMARKS:
             case URL_MATCH_API_BOOKMARK:
-                return "vnd.android.cursor.dir/bookmark";
+                return BROWSER_CONTRACT_BOOKMARK_CONTENT_TYPE;
             case URI_MATCH_BOOKMARKS_ID:
             case URL_MATCH_API_BOOKMARK_ID:
-                return "vnd.android.cursor.item/bookmark";
+                return BROWSER_CONTRACT_BOOKMARK_CONTENT_ITEM_TYPE;
             case URL_MATCH_API_SEARCHES:
-                return "vnd.android.cursor.dir/searches";
+                return BROWSER_CONTRACT_SEARCH_CONTENT_TYPE;
             case URL_MATCH_API_SEARCHES_ID:
-                return "vnd.android.cursor.item/searches";
+                return BROWSER_CONTRACT_SEARCH_CONTENT_ITEM_TYPE;
             case URL_MATCH_API_HISTORY_CONTENT:
                 return BROWSER_CONTRACT_HISTORY_CONTENT_TYPE;
             case URL_MATCH_API_HISTORY_CONTENT_ID: