Restructure protobuf targets to fit with downstream requirements.

The paths of the protobuf output files needs to match the actual tree path
in order for Bazel builds to work.

BUG=webrtc:6412
NOTRY=True

Review-Url: https://codereview.webrtc.org/2716413004
Cr-Commit-Position: refs/heads/master@{#16912}

webrtc/modules/audio_processing/BUILD.gn

@@ -794,7 +794,7 @@ if (rtc_include_tests) {
       sources = [
         "test/unittest.proto",
       ]
-      proto_out_dir = "webrtc/modules/audio_processing"
+      proto_out_dir = "webrtc/modules/audio_processing/test"
     }
 
     rtc_static_library("audioproc_protobuf_utils") {

webrtc/modules/audio_processing/audio_processing_unittest.cc

@@ -41,7 +41,7 @@ RTC_PUSH_IGNORING_WUNDEF()
 #ifdef WEBRTC_ANDROID_PLATFORM_BUILD
 #include "external/webrtc/webrtc/modules/audio_processing/test/unittest.pb.h"
 #else
-#include "webrtc/modules/audio_processing/unittest.pb.h"
+#include "webrtc/modules/audio_processing/test/unittest.pb.h"
 #endif
 RTC_POP_IGNORING_WUNDEF()