[ios] - Write runtime_deps file at 'gn gen' time.

The iOS testing templates are not generating .runtime_deps files at
'gn gen' time and this is a pre-requisite to be able to switch WebRTC
tests from the old isolate procedure the the one based on GN/MB.

Bug: webrtc:11930
Change-Id: I2cfde6a9f47526c9e5f0c6a2dbfc470bf4d3284f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407898Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806689}

testing/test.gni

@@ -249,6 +249,7 @@ template("test") {
     }
   } else if (is_ios) {
     assert(!defined(invoker.use_xvfb) || !invoker.use_xvfb)
+    _runtime_deps_file = "$root_out_dir/${target_name}.runtime_deps"
 
     declare_args() {
       # Keep the unittest-as-xctest functionality defaulted to off until the
@@ -335,6 +336,7 @@ template("test") {
 
       # Include the generate_wrapper as part of data_deps
       data_deps += [ ":${_wrapper_output_name}" ]
+      write_runtime_deps = _runtime_deps_file
     }
   } else if (is_chromeos && cros_board != "") {
     assert(!defined(invoker.use_xvfb) || !invoker.use_xvfb)