Without the usage of the srcjar_deps attribute we were not able to
include .java files from other packages without violating the package
boundary contraint.
As an example, in this CL the target "libjingle_peerconnection_java" was
directly including .java files from another packages in its "java_files"
attribute.
Using srcjar_deps we are able to declare the dependency of the target
avoiding to create hidden dependencies in the codebase.
This is not fixing the webrtc:6356 bug directly but it is a first step to
include ThreadUtils classes in libjingle_peerconnection_client_java.jar
again.
It seems also to be related to the chromium:648244 bug. This can be solved
if we can find a way to perform srcjar generation in the android_library
target without changing the semantic of the target.
BUG=webrtc:6356
Review-Url: https://codereview.webrtc.org/2610823002
Cr-Commit-Position: refs/heads/master@{#15914}
