Import //fetch from Web Platform Tests.

third_party/WebKit/LayoutTests/external/wpt/fetch/http-cache/README.md

Index: third_party/WebKit/LayoutTests/external/wpt/fetch/http-cache/README.md
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+## HTTP Caching Tests
+
+These tests cover HTTP-specified behaviours for caches, primarily from
+[RFC7234](http://httpwg.org/specs/rfc7234.html), but as seen through the
+lens of Fetch.
+
+A few notes:
+
+* By its nature, caching is optional; some tests expecting a response to be
+  cached might fail because the client chose not to cache it, or chose to
+  race the cache with a network request.
+
+* Likewise, some tests might fail because there is a separate document-level
+  cache that's ill-defined; see [this
+  issue](https://github.com/whatwg/fetch/issues/354).
+
+* [Partial content tests](partial.html) (a.k.a. Range requests) are not specified
+  in Fetch; tests are included here for interest only.
+
+* Some browser caches will behave differently when reloading /
+  shift-reloading, despite the `cache mode` staying the same.
+
+* At the moment, Edge doesn't appear to using HTTP caching in conjunction
+  with Fetch at all.