base: Refactor DiscardableMemoryProvider to build and be tested on all platforms.

This also moves DiscardableMemory::CreateLockedMemory implementations
to platform specific files and allows us to choose what implementation
to use at runtime.

No change in behavior, refactor only.

BUG=327516
TEST=base_unitttest --gtest_filter=DiscardableMemory*

Review URL: https://codereview.chromium.org/111713008

git-svn-id: http://src.chromium.org/svn/trunk/src/build@240924 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -638,7 +638,7 @@
           'proprietary_codecs%': 0,
         }],
 
-        ['OS=="mac"', {
+        ['OS=="mac" or OS=="ios"', {
           'native_discardable_memory%': 1,
           'native_memory_pressure_signals%': 1,
         }],