Add net::SourceStream and net::FilterSourceStream

net/docs/filter.md

+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" or "X reads data from Y". Data
+always flows from upstream to downstream:

[Randy Smith (Not in Mondays), 2016/08/31 19:22:44]

nit, suggestion: I find the last sentence redundant and a little confusing; if
the upstream/downstream metaphor is doing its job, it should be clear that data
is flowing upstream->downstream.   I think introducing the upstream/downstream
terminology in this paragraph is a good idea, but I'd instead just add a third
option to the previous sentence, something like |, or "X is downstream from Y"|.

[xunjieli, 2016/09/01 21:26:59]

Done.

+
+  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.