Add brotli content-encoding filter.

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index cc33dc19a98910b7a52ae349fe5c8fc29b078782..63a7ab82a53a21dac3543561054a666aded9a00d 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -842,6 +842,7 @@ void IOThread::Init() {
           switches::kEnableUserAlternateProtocolPorts)) {
     globals_->enable_user_alternate_protocol_ports = true;
   }
+  globals_->enable_brotli.set(ShouldEnableBrotli(command_line));
   // TODO(erikchen): Remove ScopedTracker below once http://crbug.com/466432
   // is fixed.
   tracked_objects::ScopedTracker tracking_profile13(
@@ -1137,6 +1138,8 @@ void IOThread::InitializeNetworkSessionParamsFromGlobals(
 
   globals.enable_npn.CopyToIfSet(&params->enable_npn);
 
+  globals.enable_brotli.CopyToIfSet(&params->enable_brotli);
+
   globals.enable_quic.CopyToIfSet(&params->enable_quic);
   globals.enable_quic_for_proxies.CopyToIfSet(&params->enable_quic_for_proxies);
   globals.quic_always_require_handshake_confirmation.CopyToIfSet(
@@ -1349,6 +1352,12 @@ void IOThread::ConfigureQuicGlobals(
   }
 }
 
+bool IOThread::ShouldEnableBrotli(const base::CommandLine& command_line) {
+  if (command_line.HasSwitch(switches::kDisableBrotli))
+    return false;
+  return command_line.HasSwitch(switches::kEnableBrotli);

[Ryan Sleevi, 2015/12/09 18:10:10]

Why did you go the route of double-flags? Typically we do one or the other - and
transition appropriately.
Do you anticipate needing to enable this by policy? Why or why not?
Do you anticipate needing to enable this on ChromeOS and Android? Why or why
not?
Do you have any framework for examining the error rates for places that enable
Brotli vs those that don't?

I'm concerned that this may not be the right thing; the QUIC code already has
too many command-line options and are inconsistent with Chrome's general policy
about command-line flags, so I wouldn't use that code as your model.

1) If you expect to need this on Android or ChromeOS by developers during
testing, then you should be making sure that the prefs system is being used, so
that you can flag that pref via about:flags
2) If you expect to control this via Finch (and you probably should, at the
least for the A/B analysis of network errors w/ and w/o this being offered),
then you should probably be using the base::Feature API

Note that 1 & 2 are not mutually exclusive.

[eustas, 2015/12/10 16:19:24]

Well, I think I need someone's help/advise to implement it.
To use a base:Feature/FieldTrial I need to declare it somewhere (and both
about_flags and io_thread must be able to refer this place). What would you
recommend?

+}
+
 bool IOThread::ShouldEnableQuic(const base::CommandLine& command_line,
                                 base::StringPiece quic_trial_group,
                                 bool quic_allowed_by_policy) {