Add version to install_attributes.proto

Right now, cryptohome has its own copy of the file. The goal is to get
rid of that. Therefore, this CL adds the version field used by
cryptohome. However, it changes it to optional since Chrome unit tests
don't set it (and 'required' is not recommended, anyway).

The file is mirrored to Chrome OS with the protofiles ebuild.

BUG=chromium:775087
TEST=Tryjobs

Change-Id: I0852f5f96d4d012f3e44f3f7d7560817c670e14e
Reviewed-on: https://chromium-review.googlesource.com/c/1436367
Auto-Submit: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626949}

components/policy/proto/install_attributes.proto

@@ -8,13 +8,13 @@ option optimize_for = LITE_RUNTIME;
 
 package cryptohome;
 
-// This must match install_attributes.proto in the Chromium OS cryptohome
-// repository. Only the fields that are relevant to the browser are present
-// here.
 message SerializedInstallAttributes {
   message Attribute {
     required string name = 1;
     required bytes value = 2;
   }
+  // Specifies the version of the install attributes used to write the file.
+  // Should be incremented when fallback behavior is needed.
+  optional uint32 version = 1 [default = 1];
   repeated Attribute attributes = 2;
 }