Fix modules_unittests on iOS.

Some of the module tests were failing on iOS, causing them to be ignored
on the trybots. Specifically this CL:
- Skips some tests that should not run on the simulator
- Fixes iOS file system test helpers
- Fixes failing nalu parser unittest

BUG=webrtc:4752
R=henrika@webrtc.org, peah@webrtc.org, sprang@webrtc.org
TBR=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/2352063002 .

Cr-Commit-Position: refs/heads/master@{#14472}
diff --git a/webrtc/test/testsupport/fileutils.cc b/webrtc/test/testsupport/fileutils.cc
index ef8e5d5..fb66a1a 100644
--- a/webrtc/test/testsupport/fileutils.cc
+++ b/webrtc/test/testsupport/fileutils.cc
@@ -45,6 +45,7 @@
 #if defined(WEBRTC_IOS)
 // Defined in iosfileutils.mm.  No header file to discourage use elsewhere.
 std::string IOSOutputPath();
+std::string IOSRootPath();
 std::string IOSResourcePath(std::string name, std::string extension);
 #endif
 
@@ -119,6 +120,9 @@
 #else // WEBRTC_ANDROID
 
 std::string ProjectRootPath() {
+#if defined(WEBRTC_IOS)
+  return IOSRootPath();
+#else
   std::string path = WorkingDir();
   if (path == kFallbackPath) {
     return kCannotFindProjectRootDir;
@@ -141,6 +145,7 @@
   }
   fprintf(stderr, "Cannot find project root directory!\n");
   return kCannotFindProjectRootDir;
+#endif
 }
 
 std::string OutputPath() {