Fix Heap.cpp dependencies.

https://codereview.chromium.org/2084933002 added a dependency on
RuntimeEnabledFeatures.h, but the target doesn't depend on the generated
files.

BUG=none

Review-Url: https://codereview.chromium.org/2393363003
Cr-Commit-Position: refs/heads/master@{#423781}

third_party/WebKit/Source/platform/heap/BUILD.gn

@@ -65,6 +65,7 @@ source_set("heap") {
 
   deps = [
     "//base",
+    "//third_party/WebKit/Source/platform:make_platform_generated",
     "//third_party/WebKit/Source/platform/heap/asm",
     "//third_party/icu",
     "//v8",