Add brotli content-encoding filter.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 905116638b554d7c56ee1436b923cde20ab5f42b..ae73b8b2fd6d1ee1a074ee64f9c9ce6e8e691712 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,
+
       # Enable FILE support by default.
       'disable_file_support%': 0,
 
@@ -865,6 +868,7 @@
 
         ['OS=="ios"', {
           'configuration_policy%': 0,
+          'disable_brotli_support%': 1,
           'disable_ftp_support%': 1,
           'enable_extensions%': 0,
           'cld2_table_size%': 0,
@@ -1222,6 +1226,7 @@
     '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'],
       }],