Re-land: base: Replace PurgeAndTruncate with Shrink function.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 81519cca3177f8d8d3e7a22f20d6777c694b3c89..51a8576eefd90330fc8059b5e4e8fe50c4d66aad 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -606,6 +606,9 @@
       # Platform sends memory pressure signals natively.
       'native_memory_pressure_signals%': 0,
 
+      # Platform supports shrinking of discardable shared memory segments.
+      'discardable_shared_memory_shrinking%': 0,
+
       'spdy_proxy_auth_property%' : '',
       'spdy_proxy_auth_value%' : '',
       'enable_mdns%' : 0,
@@ -777,6 +780,10 @@
           'native_memory_pressure_signals%': 1,
         }],
 
+        ['OS!="win" and OS!="android"', {
+          'discardable_shared_memory_shrinking%': 1,
+        }],
+
         # Enable autofill dialog for Android, Mac and Views-enabled platforms.
         ['toolkit_views==1 or (OS=="android" and android_webview_build==0) or OS=="mac"', {
           'enable_autofill_dialog%': 1,
@@ -1198,6 +1205,7 @@
     'google_default_client_secret%': '<(google_default_client_secret)',
     'enable_supervised_users%': '<(enable_supervised_users)',
     'native_memory_pressure_signals%': '<(native_memory_pressure_signals)',
+    'discardable_shared_memory_shrinking%': '<(discardable_shared_memory_shrinking)',
     'spdy_proxy_auth_property%': '<(spdy_proxy_auth_property)',
     'spdy_proxy_auth_value%': '<(spdy_proxy_auth_value)',
     'enable_mdns%' : '<(enable_mdns)',
@@ -2715,6 +2723,9 @@
       ['native_memory_pressure_signals==1', {
         'defines': ['SYSTEM_NATIVELY_SIGNALS_MEMORY_PRESSURE'],
       }],
+      ['discardable_shared_memory_shrinking==1', {
+        'defines': ['DISCARDABLE_SHARED_MEMORY_SHRINKING'],
+      }],
       ['use_udev==1', {
         'defines': ['USE_UDEV'],
       }],