Enable LayoutNGPaintFragments on build bot linux_layout_tests_layout_ng

This patch enables LayoutNGPaintFragments and slimming-paint v175
on linux_layout_tests_layout_ng.

Following build# from layout_ng bots are included:
2423 2428

Still more flaky results are expected for layout_ng bots. I will
maintain the FlagExpectations when this is landed.

Bug: 792329, 714962
Change-Id: I2bca54943a1db545be3e33059ac55091ac885fa4
Reviewed-on: https://chromium-review.googlesource.com/813139Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522733}

testing/buildbot/chromium.fyi.json

@@ -7076,7 +7076,9 @@
     "isolated_scripts": [
       {
         "args": [
-          "--additional-driver-flag=--enable-blink-features=LayoutNG"
+          "--additional-driver-flag=--enable-blink-features=LayoutNG",
+          "--additional-driver-flag=--enable-blink-features=LayoutNGPaintFragments",
+          "--additional-driver-flag=--enable-slimming-paint-v175"
         ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {