Move InitClassLoader from JNI_OnLoad to LoadGlobalClassReferenceHolder

This fixes clients with custom JNI_OnLoad code. This is basically a
reland of https://webrtc-review.googlesource.com/21281, but called from
LoadGlobalClassReferenceHolder instead of InitGlobalJniVariables. The
problem it solves is that some clients call InitGlobalJniVariables
from a strange thread and without using the rest of our Java API.

TBR=lliuu@@webrtc.org,sakal@webrtc.org

Bug: webrtc:8278
Change-Id: I664e50920bcd7e087ec2d8bb5be13569e8ff3088
Reviewed-on: https://webrtc-review.googlesource.com/22801
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20663}
diff --git a/sdk/android/src/jni/classreferenceholder.cc b/sdk/android/src/jni/classreferenceholder.cc
index 0aa3e62..29295b6 100644
--- a/sdk/android/src/jni/classreferenceholder.cc
+++ b/sdk/android/src/jni/classreferenceholder.cc
@@ -9,6 +9,7 @@
  */
 #include "sdk/android/src/jni/classreferenceholder.h"
 
+#include "sdk/android/src/jni/class_loader.h"
 #include "sdk/android/src/jni/jni_helpers.h"
 
 namespace webrtc {
@@ -34,8 +35,12 @@
 static ClassReferenceHolder* g_class_reference_holder = nullptr;
 
 void LoadGlobalClassReferenceHolder() {
+  JNIEnv* env = GetEnv();
   RTC_CHECK(g_class_reference_holder == nullptr);
-  g_class_reference_holder = new ClassReferenceHolder(GetEnv());
+  g_class_reference_holder = new ClassReferenceHolder(env);
+  // TODO(magjed): This is a weird place to call the other class loader from,
+  // but the only place that will keep backwards compatibility.
+  InitClassLoader(env);
 }
 
 void FreeGlobalClassReferenceHolder() {