Add brotli content-encoding filter.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index b81acbe25aecc1db418a72e93bbbc55600b81629..4e8fb52c46ca746e2ba690b4248ab318af9c41b8 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -597,6 +597,9 @@
       # Enables used resource whitelist generation; disabled by default.
       'enable_resource_whitelist_generation%': 0,
 
+      # Enable BROTLI support by default.
+      'disable_brotli_support%': 0,

[Ryan Sleevi, 2015/12/17 01:04:40]

There is strong resistance towards adding system-wide flags like this.

Can you explain more why this is necessary, and whether you see this as
short-term or long-term?

[eustas, 2015/12/17 16:44:54]

This flag was added in response to a request to make it possible completely
disable brotli in a build.
So I've decided to mimic FTP and File support.
I'm OK to remove it (it was planned as a short-term, until feature is fully
launched).

[Ryan Sleevi, 2015/12/17 22:49:33]

Right, we're generally trying not to add build flags like that, since every
permutation adds to complexity and testing.

For example, it's unclear which is "supported" or not, what bots we have
building either, etc.

+
       # Enable FILE support by default.
       'disable_file_support%': 0,
 
@@ -858,6 +861,7 @@
 
         ['OS=="ios"', {
           'configuration_policy%': 0,
+          'disable_brotli_support%': 1,
           'disable_ftp_support%': 1,
           'enable_extensions%': 0,
           'cld2_table_size%': 0,
@@ -1218,6 +1222,7 @@
     'cld_version%': '<(cld_version)',
     'cld2_table_size%': '<(cld2_table_size)',
     'enable_captive_portal_detection%': '<(enable_captive_portal_detection)',
+    'disable_brotli_support%': '<(disable_brotli_support)',
     'disable_file_support%': '<(disable_file_support)',
     'disable_ftp_support%': '<(disable_ftp_support)',
     'enable_task_manager%': '<(enable_task_manager)',
@@ -3037,6 +3042,9 @@
       ['enable_settings_app==1', {
         'defines': ['ENABLE_SETTINGS_APP=1'],
       }],
+      ['disable_brotli_support==1', {
+        'defines': ['DISABLE_BROTLI_SUPPORT=1'],
+      }],
       ['disable_file_support==1', {
         'defines': ['DISABLE_FILE_SUPPORT=1'],
       }],