Fabric: Calling JSVM GC on memory pressure event on iOS

Summary:
This change is especially important for Fabric when a lot of objects (mostly `ShadowNode`s) have shared ownership. Without this change, JSVM could not know that bunch of natively allocated objects should be deallocated.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: dulinriley

Differential Revision: D21484773

fbshipit-source-id: 46e32de0f108082e60df346884c9287023156149

React/CxxBridge/RCTCxxBridge.mm

@@ -241,10 +241,20 @@ - (instancetype)initWithParentBridge:(RCTBridge *)bridge
     _moduleDataByID = [NSMutableArray new];
 
     [RCTBridge setCurrentBridge:self];
+
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(handleMemoryWarning)
+                                                 name:UIApplicationDidReceiveMemoryWarningNotification
+                                               object:nil];
   }
   return self;
 }
 
+- (void)dealloc
+{
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
 + (void)runRunLoop
 {
   @autoreleasepool {
@@ -278,6 +288,13 @@ - (void)_tryAndHandleError:(dispatch_block_t)block
   }
 }
 
+- (void)handleMemoryWarning
+{
+  if (_reactInstance) {
+    _reactInstance->handleMemoryPressure(15 /* TRIM_MEMORY_RUNNING_CRITICAL */);
+  }
+}
+
 /**
  * Ensure block is run on the JS thread. If we're already on the JS thread, the block will execute synchronously.
  * If we're not on the JS thread, the block is dispatched to that thread. Any errors encountered while executing