Fixing WebRTC after moving from src/webrtc to src/

In https://webrtc-review.googlesource.com/c/src/+/1560 we moved WebRTC
from src/webrtc to src/ (in order to preserve an healthy git history).
This CL takes care of fixing header guards, #include paths, etc...

NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
TBR=tommi@webrtc.org


Bug: chromium:611808
Change-Id: Iea91618212bee0af16aa3f05071eab8f93706578
Reviewed-on: https://webrtc-review.googlesource.com/1561
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19846}
diff --git a/.gn b/.gn
index 7591ffb..882d220 100644
--- a/.gn
+++ b/.gn
@@ -20,7 +20,29 @@
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
-check_targets = [ "//webrtc/*" ]
+check_targets = [
+  "//api/*",
+  "//audio/*",
+  "//backup/*",
+  "//call/*",
+  "//common_audio/*",
+  "//common_video/*",
+  "//examples/*",
+  "//logging/*",
+  "//media/*",
+  "//modules/*",
+  "//ortc/*",
+  "//p2p/*",
+  "//pc/*",
+  "//rtc_base/*",
+  "//rtc_tools/*",
+  "//sdk/*",
+  "//stats/*",
+  "//system_wrappers/*",
+  "//test/*",
+  "//video/*",
+  "//voice_engine/*",
+]
 
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly