Design doc as reference:
https://docs.google.com/a/google.com/document/d/1o-_qMYEJLgqS13_KV7OVqqrEEwNgk3O41OosD53dpo4/edit?usp=sharing

As @sgurun suggested, it's better to change the default handlers for generic features rather than add a new custom handler, since it prevents code duplication in case another feature needs it as well.
Should modify the default handlers for those features (e.g. echoing multiple headers).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4672b3b6953050cf8a9fd4ba54bb511c24f16d8c

commit 4672b3b6953050cf8a9fd4ba54bb511c24f16d8c
Author: shenghuazhang <shenghuazhang@chromium.org>
Date: Thu Feb 16 03:24:56 2017

[Embedded Test Server] Enable handler HandleEchoHeader echoing multiple headers

The default handler HandleEchoHeader allows single header value setting in the
http response when calls /echoheader. This CL enables the handler
HandleEchoHeader echoing multiple headers.

BUG= 686247 

Review-Url: https://codereview.chromium.org/2694923006
Cr-Commit-Position: refs/heads/master@{#450857}

[modify] https://crrev.com/4672b3b6953050cf8a9fd4ba54bb511c24f16d8c/net/test/embedded_test_server/default_handlers.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b2fdbb4b96e498a7100bc07738f5406555bf466

commit 2b2fdbb4b96e498a7100bc07738f5406555bf466
Author: shenghuazhang <shenghuazhang@chromium.org>
Date: Wed Jun 14 00:30:37 2017

[Android Webview] Refactor LoadUrlTest and work on embedded_test_server custom handler to support the java code.

Some tests in LoadUrlTest needs embedded_test_server custom handler.
Here to refactor the custom handler related tests, and implement custom
handler in embedded_test_server native code, and make it support
java code. (See  bug 686247 )

BUG= 686247 

Review-Url: https://codereview.chromium.org/2687573002
Cr-Commit-Position: refs/heads/master@{#479227}

[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/javatests/src/org/chromium/android_webview/test/AcceptLanguageTest.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/javatests/src/org/chromium/android_webview/test/AwContentsTest.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/javatests/src/org/chromium/android_webview/test/AwSettingsTest.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/javatests/src/org/chromium/android_webview/test/AwTestBase.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/javatests/src/org/chromium/android_webview/test/LoadUrlTest.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/BUILD.gn
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/BUILD.gn
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/aw_embedded_test_server.cc
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/aw_embedded_test_server.h
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/aw_test_entry_point.cc
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/aw_test_jni_onload.cc
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/aw_test_jni_onload.h
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/java/AndroidManifest.xml
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/java/src/org/chromium/android_webview/test/AwEmbeddedTestServer.java
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/java/src/org/chromium/android_webview/test/AwEmbeddedTestServerImpl.java
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/android_webview/test/embedded_test_server/java/src/org/chromium/android_webview/test/AwEmbeddedTestServerService.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/android/javatests/src/org/chromium/net/test/EmbeddedTestServer.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/android/javatests/src/org/chromium/net/test/EmbeddedTestServerImpl.java
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/android/javatests/src/org/chromium/net/test/IEmbeddedTestServerInterface.aidl
[add] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/android/javatests/src/org/chromium/net/test/OWNERS
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/embedded_test_server/android/embedded_test_server_android.cc
[modify] https://crrev.com/2b2fdbb4b96e498a7100bc07738f5406555bf466/net/test/embedded_test_server/android/embedded_test_server_android.h