Allowing ${executable_name} in executable target.

An executable will implicitly have a target created of the exe name if
none was explicitly added. When exe's are defined in in a list and they
had a target then that prevented the exe name from also being a target.
Allowing this variable allows this to work.
Runs can now have "-commands" to remove commands from exe's.

If a run wants to remove a command from an executable then it can now
specify this.
out -> ${out_dir}
crbuild: Added content_shell executable.
crbuild: Added clusterfuzz config for chrome.
crbuild: Added env_chromium_unittests_apk
git-rebaseall: Warning with parent branch doesn't exist.
Added idb-content-browsertests-android
crbuild: all_webkit -> blink_tests
crbuild: webkit_unit_tests depends on xdisplaycheck
crbuild: Tweaked names of layout test targets (webkit->blink)