Expose computation of md5 content checksums for files via a file manager private API.

chrome/common/extensions/api/file_manager_private.idl

Index: chrome/common/extensions/api/file_manager_private.idl
diff --git a/chrome/common/extensions/api/file_manager_private.idl b/chrome/common/extensions/api/file_manager_private.idl
index a1bad67918f1f3e503d5b4a31232e59cf242e54e..6558b3f47f0ae20f851b2555bd7f719fe3f2a878 100644
--- a/chrome/common/extensions/api/file_manager_private.idl
+++ b/chrome/common/extensions/api/file_manager_private.idl
@@ -557,6 +557,9 @@ callback GetProfilesCallback = void(ProfileInfo[] profiles,
 callback ResolveEntriesCallback =
     void([instanceOf=FileEntry] object[] entries);
 
+// |checksum| Result checksum.
+callback ComputeChecksumCallback = void(DOMString checksum);
+
 interface Functions {
   // Logout the current user for navigating to the re-authentication screen for
   // the Google account.
@@ -807,6 +810,12 @@ interface Functions {
   // Opens inspector window.
   // |type| InspectionType which specifies how to open inspector.
   static void openInspector(InspectionType type);
+
+  // Computes an MD5 checksum for the given file.
+  // |fileURL| The URL of the file to checksum.
+  // |callback|
+  static void computeChecksum(DOMString fileURL,
+                              ComputeChecksumCallback callback);
 };
 
 interface Events {