Add discardable memory emulation for non-android/mac platforms

Adds support for emulated discardable memory. The memory is managed by a
provider which listens for memory pressure notifications from the platform.

Currently, only android pushes these notifications, but in future patches, we
will apply pressure on other platforms in certain situations (e.g., when a tab
gets backgrounded).

BUG=237681

Review URL: https://codereview.chromium.org/17106004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@231845 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -557,6 +557,11 @@
           'proprietary_codecs%': 0,
         }],
 
+        ['OS=="mac"', {
+          'native_discardable_memory%': 1,
+          'native_memory_pressure_signals%': 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
@@ -2057,6 +2062,12 @@
       ['enable_hidpi==1', {
         'defines': ['ENABLE_HIDPI=1'],
       }],
+      ['native_discardable_memory==1', {
+        'defines': ['DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY'],
+      }],
+      ['native_memory_pressure_signals==1', {
+        'defines': ['SYSTEM_NATIVELY_SIGNALS_MEMORY_PRESSURE'],
+      }],
       ['fastbuild!=0', {
         'xcode_settings': {
           'GCC_GENERATE_DEBUGGING_SYMBOLS': 'NO',
@@ -3467,12 +3478,6 @@
               '-B<(PRODUCT_DIR)/../../third_party/gold',
             ],
           }],
-          ['native_discardable_memory', {
-            'defines': ['DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY'],
-          }],
-          ['native_memory_pressure_signals', {
-            'defines': ['SYSTEM_NATIVELY_SIGNALS_MEMORY_PRESSURE'],
-          }],
         ],
       },
     }],