Add net::SourceStream and net::FilterSourceStream

net/docs/filter.md

Index: net/docs/filter.md
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+The network stack implements support for Content-Encodings using
+"source streams", which can be composed together and mutate all the incoming
+bytes from a URLRequestJob. Currently, the following streams are implemented:
+
+* gzip (handling "deflate" and "gzip" Content-Encodings)
+* sdch (handling "sdch" Content-Encoding)
+* brotli (handling "br" Content-Encoding)
+
+Source streams conceptually form a chain, with the URLRequestJob as both the
+beginning and end of the chain, meaning the URLRequestJob produces raw bytes at
+the end and consumes unencoded bytes at the beginning. For example, to support a
+hypothetical "Content-Encoding: bar,foo", streams would be arranged like so,
+with "X <-- Y" meaning "data flows from Y to X", "X reads data from Y", or
+"X is downstream from Y".
+
+  URLRequestJob <-- BarSourceStream <-- FooSourceStream <-- URLRequestJob
+                                                       (URLRequestSourceStream)
+
+Here URLRequestJob pulls filtered bytes from its upstream, BarSourceStream,
+which pulls filtered bytes from FooSourceStream, which in turn pulls raw bytes
+from URLRequestJob. URLRequestSourceStream is the ultimate upstream, which
+produces the raw data. Every stream in the chain owns its upstream. In this
+case, the ultimate downstream, URLRequestJob, owns its upstream,
+BarSourceStream, which in turn owns FooSourceStream. FooSourceStream owns its
+upstream, URLRequestSourceStream.
+
+All source streams conform to the following interface (named SourceStream in the
+tree):
+
+  int Read(IOBuffer* dest_buffer, size_t buffer_size,
+           const OnReadCompleteCallback& callback);
+
+This function can return either synchronously or asynchronously via the supplied
+callback. The source stream chain is "pull-based", in that data does not
+propagate through the chain until requested by the final consumer of the
+filtered data.