Revert "Move InitClassLoader from JNI_OnLoad to InitGlobalJniVariables."

This reverts commit b3a8c980f7b33e7457f9a1ee3582ccbc28b7f70c.

Reason for revert: Breaking internal projects

Original change's description:
> Move InitClassLoader from JNI_OnLoad to InitGlobalJniVariables.
> 
> This fixes clients with custom JNI_OnLoad code.
> 
> Bug: webrtc:8278
> Change-Id: I67b45c2ca07b13b0bf968c5ac17ec6f1bcc58a86
> Reviewed-on: https://webrtc-review.googlesource.com/21281
> Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
> Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20629}

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: webrtc:8278
Change-Id: I6add8182172501cb93bed998f68ce848c7280a98
Reviewed-on: https://webrtc-review.googlesource.com/22780
Reviewed-by: Lu Liu <lliuu@webrtc.org>
Commit-Queue: Lu Liu <lliuu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20660}
diff --git a/sdk/android/src/jni/jni_helpers.cc b/sdk/android/src/jni/jni_helpers.cc
index f55cd1b..4c3724f 100644
--- a/sdk/android/src/jni/jni_helpers.cc
+++ b/sdk/android/src/jni/jni_helpers.cc
@@ -15,7 +15,6 @@
 #include <unistd.h>
 #include <vector>
 
-#include "sdk/android/src/jni/class_loader.h"
 #include "sdk/android/src/jni/classreferenceholder.h"
 
 namespace webrtc {
@@ -79,8 +78,6 @@
   if (jvm->GetEnv(reinterpret_cast<void**>(&jni), JNI_VERSION_1_6) != JNI_OK)
     return -1;
 
-  InitClassLoader(jni);
-
   return JNI_VERSION_1_6;
 }
 
diff --git a/sdk/android/src/jni/jni_onload.cc b/sdk/android/src/jni/jni_onload.cc
index d70d7b8..cb708e7 100644
--- a/sdk/android/src/jni/jni_onload.cc
+++ b/sdk/android/src/jni/jni_onload.cc
@@ -28,6 +28,8 @@
 
   RTC_CHECK(rtc::InitializeSSL()) << "Failed to InitializeSSL()";
   LoadGlobalClassReferenceHolder();
+  JNIEnv* env = AttachCurrentThreadIfNeeded();
+  InitClassLoader(env);
 
   return ret;
 }