Imply SlimmingPaintV175 when LayoutNGPaintFragments is enabled

As suggested by chrishtr@ at:
https://groups.google.com/a/chromium.org/d/msg/layout-dev/O_D8ylaEyyE/wCiDNT8lCgAJ

Bug: 714962
Change-Id: Ic53ff4c3f492e8201c4582fe2c7e497ee6b10050
Reviewed-on: https://chromium-review.googlesource.com/821393Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524670}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -970,7 +970,7 @@
     },
     {
       name: "SlimmingPaintV175",
-      implied_by: ["SlimmingPaintV2"],
+      implied_by: ["SlimmingPaintV2", "LayoutNGPaintFragments"],
     },
     {
       name: "SlimmingPaintV2",