Add http/tests/devtools/elements/shadow to virtual/incrmental-shadow-dom tests

http/tests/devtools/elements/shadow is running several tests for shadow dom's distribution
used in devtools.

shadow-distribution.js is failing with the IncrementalShadowDOM flag.

TBR=kochi,rakina

Bug: 776656
Change-Id: Id780279b397c91bf3af7136727d36f83e1179565
Reviewed-on: https://chromium-review.googlesource.com/1029392
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553943}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1180,6 +1180,7 @@ crbug.com/824918 virtual/layout_ng_experimental/printing/width-overflow.html [ F
 # ====== IncrementalShadowDOM-only failures from here ======
 
 crbug.com/776656 virtual/incremental-shadow-dom/external/wpt/shadow-dom/untriaged/styles/test-003.html [ Failure ]
+crbug.com/776656 virtual/incremental-shadow-dom/http/tests/devtools/elements/shadow/shadow-distribution.js [ Failure ]
 
 # ====== IncrementalShadowDOM-only failures until here ======
 

third_party/WebKit/LayoutTests/VirtualTestSuites

@@ -604,6 +604,11 @@
     "base": "html/details_summary",
     "args": ["--enable-blink-features=SlotInFlatTree,IncrementalShadowDOM"]
   },
+  {
+    "prefix": "incremental-shadow-dom",
+    "base": "http/tests/devtools/elements/shadow",
+    "args": ["--enable-blink-features=SlotInFlatTree,IncrementalShadowDOM"]
+  },
   {
     "prefix": "incremental-shadow-dom",
     "base": "external/wpt/css/css-scoping",

third_party/WebKit/LayoutTests/virtual/incremental-shadow-dom/http/tests/devtools/elements/shadow/README.txt

+# This suite runs the tests in http/tests/devtools/elements/shadow with
+# --enable-blink-features=IncrementalShadowDOM.
+# See crbug.com/776656 for details.