Move tools/mb -> tools-webrtc/mb

In addition to moving tools/mb -> tools-webrtc/mb, also
move webrtc/build/mb_config.pyl into tools-webrtc/mb
to match the default location better.
Remove Chromium-specific check for 'mb validate' that failed
due to doing this (we never cleaned that when we forked the code).

BUG=webrtc:5006
TBR=ehmaldonado@webrtc.org
NOTRY=True
TESTED=Manually ran:
tools-webrtc/mb/mb.py gen -m client.webrtc -b 'iOS64 Release' --config-file tools-webrtc/mb/mb_config.pyl  //out/Release-iphoneos
tools-webrtc/mb/mb.py gen -m client.webrtc -b 'Mac64 Release' --config-file tools-webrtc/mb/mb_config.pyl  //out/Release-mac

Review-Url: https://codereview.webrtc.org/2585743002 .
Cr-Commit-Position: refs/heads/master@{#15664}

PRESUBMIT.py

@@ -459,6 +459,7 @@ def _CommonChecks(input_api, output_api):
                   r'^tools[\\\/]valgrind[\\\/].*\.py$',
                   r'^tools[\\\/]win[\\\/].*\.py$',
                   # TODO(phoglund): should arguably be checked.
+                  r'^tools-webrtc[\\\/]mb[\\\/].*\.py$',
                   r'^tools-webrtc[\\\/]valgrind[\\\/].*\.py$',
                   r'^xcodebuild.*[\\\/].*\.py$',),
       disabled_warnings=['F0401',  # Failed to import x