Remove Blink platform heap GYP integration.

Inline the source lists into the GN build.

Move blink_heap_unittests from the platform directory to the heap directory.

Review-Url: https://codereview.chromium.org/2298443003
Cr-Commit-Position: refs/heads/master@{#415835}

diff --git a/BUILD.gn b/BUILD.gn
index 298b2b6..9ab0ee0 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -287,8 +287,8 @@
       "//mojo/edk/test:mojo_public_system_unittests",
       "//net:net_perftests",
       "//services/shell/public/cpp",
-      "//third_party/WebKit/Source/platform:blink_heap_unittests",
       "//third_party/WebKit/Source/platform:blink_platform_unittests",
+      "//third_party/WebKit/Source/platform/heap:blink_heap_unittests",
       "//third_party/WebKit/Source/web:webkit_unit_tests",
       "//third_party/WebKit/Source/wtf:wtf_unittests",
       "//third_party/catapult/telemetry:bitmaptools($host_toolchain)",
@@ -1115,8 +1115,8 @@
       "//remoting:remoting_unittests",
       "//skia:skia_unittests",
       "//sql:sql_unittests",
-      "//third_party/WebKit/Source/platform:blink_heap_unittests",
       "//third_party/WebKit/Source/platform:blink_platform_unittests",
+      "//third_party/WebKit/Source/platform/heap:blink_heap_unittests",
       "//third_party/angle/src/tests:angle_unittests",
       "//third_party/cacheinvalidation:cacheinvalidation_unittests",
       "//third_party/leveldatabase:env_chromium_unittests",