Add memory pressure dispatching callback to MemoryCoordinator

components/memory_coordinator/browser/BUILD.gn

Index: components/memory_coordinator/browser/BUILD.gn
diff --git a/components/memory_coordinator/browser/BUILD.gn b/components/memory_coordinator/browser/BUILD.gn
index 836ef4b412aa01ca1860f86de3d0ebe8d97a7999..0afa65eb2142794517d5a8f7444f57fbd15b1538 100644
--- a/components/memory_coordinator/browser/BUILD.gn
+++ b/components/memory_coordinator/browser/BUILD.gn
@@ -17,3 +17,16 @@ static_library("browser") {
     "//components/memory_coordinator/common",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+
+  sources = [
+    "memory_coordinator_unittest.cc",
+  ]
+
+  deps = [
+    ":browser",
+    "//testing/gtest",
+  ]
+}