Refactor safe browsing service tests.

Extracts code which helps to mock communication with the V4 database. This code will be reused in the browser tests for the Subrsource Filter.

Part 1, https://codereview.chromium.org/2690413002 (this one)
Part 2, https://codereview.chromium.org/2645283007

BUG=671962

Review-Url: https://codereview.chromium.org/2690413002
Cr-Commit-Position: refs/heads/master@{#453372}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 00e9a117..8c7dc2b 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -2048,6 +2048,8 @@
       "safe_browsing/threat_details_history.h",
       "safe_browsing/ui_manager.cc",
       "safe_browsing/ui_manager.h",
+      "safe_browsing/v4_test_utils.cc",
+      "safe_browsing/v4_test_utils.h",
     ]
     deps += [
       "//chrome/browser/safe_browsing:chunk_proto",