Adds blink_tests -> mojo test interfaces runtime dependency.

This is so that mojo/binding.html can find the generated bindings for
math_calculator.mojom. This layout test was added in r455925, but failed
to add this data dependency.

BUG=700281
TEST=gn desc blink_tests runtime_deps

Review-Url: https://codereview.chromium.org/2742793003
Cr-Commit-Position: refs/heads/master@{#456119}

BUILD.gn

@@ -897,6 +897,7 @@ if (!is_ios) {
     testonly = true
     data_deps = [
       "//content/shell:content_shell",
+      "//mojo/public/interfaces/bindings/tests:test_interfaces",
       "//third_party/WebKit/public:blink_devtools_frontend_resources_files",
       "//third_party/mesa:osmesa",
       "//tools/imagediff",