Add support for loading API keys from Info.plist on iOS and macOS.

google_apis/BUILD.gn

Index: google_apis/BUILD.gn
diff --git a/google_apis/BUILD.gn b/google_apis/BUILD.gn
index 4ce63ba66d30c87e1ade46431f92c188458d3aa3..a129ae77b2e8954d926e5c2c32c192a83d9ab0a8 100644
--- a/google_apis/BUILD.gn
+++ b/google_apis/BUILD.gn
@@ -173,6 +173,17 @@ template("google_apis_tmpl") {
         "drive/time_util.h",
       ]
     }
+
+    if (is_mac || is_ios) {
+      set_sources_assignment_filter([])
+
+      sources += [
+        "google_api_keys_mac.h",
+        "google_api_keys_mac.mm",
+      ]
+
+      set_sources_assignment_filter(sources_assignment_filter)
+    }
   }
 }
 
@@ -260,4 +271,12 @@ test("google_apis_unittests") {
       "drive/time_util_unittest.cc",
     ]
   }
+
+  if (is_mac || is_ios) {
+    set_sources_assignment_filter([])
+    sources += [ "google_api_keys_mac_unittest.mm" ]
+    set_sources_assignment_filter(sources_assignment_filter)
+
+    deps += [ "//third_party/ocmock" ]
+  }
 }