[ios] Adds an NSPrincipalClass entry to the XCTest Info.plist.

This defaults to NSObject, but can be set to a class that implements the
XCTestObservation protocol to perform one-time setup in XCTest bundles.

BUG=None

Change-Id: I3ff7abbf928a16ce6626fc99ed8fd747902cc791
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617623Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660845}

build/config/ios/Module-Info.plist

@@ -20,5 +20,7 @@
   <string>????</string>
   <key>CFBundleVersion</key>
   <string>1</string>
+  <key>NSPrincipalClass</key>
+  <string>${XCTEST_BUNDLE_PRINCIPAL_CLASS}</string>
 </dict>
 </plist>

build/config/ios/rules.gni

@@ -1660,14 +1660,23 @@ template("ios_xctest_bundle") {
       info_plist = "//build/config/ios/Module-Info.plist"
       executable_name = _output_name
 
+      if (defined(invoker.xctest_bundle_principal_class)) {
+        _principal_class = invoker.xctest_bundle_principal_class
+      } else {
+        # Fall back to a reasonable default value.
+        _principal_class = "NSObject"
+      }
+      extra_substitutions =
+          [ "XCTEST_BUNDLE_PRINCIPAL_CLASS=${_principal_class}" ]
+
       if (ios_automatically_manage_certs) {
         # Use a fixed bundle identifier for EarlGrey tests when using Xcode to
         # manage the certificates as the number of free certs is limited.
-        extra_substitutions = [
+        extra_substitutions += [
           "MODULE_BUNDLE_ID=gtest.${ios_generic_test_bundle_id_suffix}-module",
         ]
       } else {
-        extra_substitutions = [ "MODULE_BUNDLE_ID=gtest.$_output_name" ]
+        extra_substitutions += [ "MODULE_BUNDLE_ID=gtest.$_output_name" ]
       }
     }
 
@@ -2027,7 +2036,11 @@ template("ios_xcuitest_test") {
 
   _xcuitest_module_output = _xcuitest_target
   ios_xctest_bundle(_xcuitest_module_target) {
-    forward_variables_from(invoker, [ "xcode_test_application_name" ])
+    forward_variables_from(invoker,
+                           [
+                             "xcode_test_application_name",
+                             "xctest_bundle_principal_class",
+                           ])
 
     product_type = _ios_xcode_xcuitest_bundle_id
     host_target = _xcuitest_runner_target

ios/third_party/earl_grey2/ios_eg2_test.gni

@@ -69,6 +69,7 @@ template("ios_eg2_test") {
     forward_variables_from(invoker,
                            [
                              "xcode_test_application_name",
+                             "xctest_bundle_principal_class",
                              "bundle_deps",
                              "deps",
                            ])