ios: Use PlatformTest instead of testing::Test

When trying to update Chromium's copy of crashpad I got this error
message:

Banned functions were used.
    third_party\crashpad\crashpad\client\crashpad_client_ios_test.mm:33:
      testing::Test should not be used in Objective-C++ code as it does
      not drain the autorelease pool at the end of the test. Use
      PlatformTest instead.

So, I'm fixing the code as requested.

The change was introduced in crrev.com/c/3418581

Change-Id: I4888febbd41b6365d9bde5ad062565770496243f
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3459403
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
diff --git a/client/crashpad_client_ios_test.mm b/client/crashpad_client_ios_test.mm
index a9a00c5..9800d40 100644
--- a/client/crashpad_client_ios_test.mm
+++ b/client/crashpad_client_ios_test.mm
@@ -30,7 +30,7 @@
 namespace test {
 namespace {
 
-class CrashpadIOSClient : public testing::Test {
+class CrashpadIOSClient : public PlatformTest {
  protected:
   // testing::Test: