Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/README.md

 # Mojo EDK (embedder development kit)
 
 The Mojo EDK is a "library" that provides implementations of the basic Mojo
 system primitives (e.g., message pipes). It is not meant for ordinary
 applications, but for _embedders_ who in turn expose the basic Mojo primitives
 to other applications/content.
 
 For example, this is used by `mojo_shell` to implement the Mojo primitives,
 which it in turn exposes to Mojo applications, and by Flutter, which exposes
 Mojo primitives to Flutter applications. (`mojo_shell` and Flutter are embedders
 of the Mojo system implementation.)
 
 Note: The embedder API is not stable (neither at the source nor at the binary
 level) and will evolve over time.
 
 ## Organization
 
 ### Subdirectories
 
 * [//mojo/edk/base_edk](base_edk): The embedder API requires various things to
   be implemented or injected by the embedder. This contains implementations of
   these things for use with [//base](../../base). (This may also be usable with
   other sufficiently-similar derivatives of Chromium's
   [//base](https://chromium.googlesource.com/chromium/src/+/master/base/).)
 * [//mojo/edk/embedder](embedder): The header files in this directory constitute
   the public API available to embedders. (The .cc files are private and mostly
   serve to bridge between the public API and the private API in
   [system](system).)
+* [//mojo/edk/platform](platform): This contains platform abstractions and
+  declarations of embedder-dependent things. Some of these must be provided by
+  the embedder, either by implementing an interface or by implementing a class
+  outright.
 * [//mojo/edk/system](system): This contains the bulk of the actual
   implementation, and is entirely private.
 * [//mojo/edk/system/test](system/test): This contains private test helpers used
   by the EDK's internal tests.
 * [//mojo/edk/test](test): In principle, this contains test helpers for use by
   embedders (but see the **TODO** below).
 * [//mojo/edk/util](util): This contains basic helpers built on top of the C++
   library and also some POSIX APIs, notably pthreads. These are used by all the
   other parts of the EDK, and are also available for embedders to use. (Outside
   its tests, it should not depend on other parts of the EDK.)
 
 ### TODO(vtl)
 
 * [//mojo/edk/test](test) currently contains things that aren't meant for
   embedders. (They can't be moved to [//mojo/edk/system/test](system/test)
   because of their dependencies, but they should be moved elsewhere instead.)
 * There should be a "platform" directory. Many of the things in
   [//mojo/edk/embedder](embedder) (especially in the `platform` target) should
   be moved here (though some of the implementations should be moved to
   [//mojo/edk/base_edk](base_edk).
 
 ## See also
 
 * [//mojo/public](../public): the Mojo public SDK