Remove remains of webrtc/base

webrtc/base/README.md

Index: webrtc/base/README.md
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+# What?
+
+The contents of base have moved to rtc_base.
+
+# Why?
+
+We want to move all the contents in the webrtc directory to top-level, because:
+* When we migrate from Rietveld to PolyGerrit we won't be able to apply WebRTC
+patches on Chromium trybots (which is a very useful feature, especially as we
+plan to have such trybots in our default trybot set).
+This is because PolyGerrit needs the repo in Chromium to be the same as the
+WebRTC repo, otherwise it won’t be able to apply the patch.
+* To fully automate rolling into Chromium DEPS, we wish to use the LKGR finder,
+but doing so is blocked on this (needs the same revision hashes to avoid ugly
+hacks). See [this bug](http://crbug.com/666726).
+
+# Tracking Bugs
+
+[Chromium tracking bug](http://crbug.com/611808)
+[WebRTC tracking bug](https://bugs.chromium.org/p/webrtc/issues/detail?id=7634)