DevTools: add CSS completion for contain property.

BUG=561713

Review-Url: https://codereview.chromium.org/2087773002
Cr-Commit-Position: refs/heads/master@{#402216}
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/CSSMetadata.js b/third_party/WebKit/Source/devtools/front_end/sdk/CSSMetadata.js
index d788204..421c6522 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/CSSMetadata.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/CSSMetadata.js
@@ -293,6 +293,9 @@
     "overflow": { values: [
         "hidden", "auto", "visible", "overlay", "scroll"
     ] },
+    "contain": { values: [
+        "none", "strict", "content", "size", "layout", "style", "paint"
+    ] },
     "text-rendering": { values: [
         "auto", "optimizeSpeed", "optimizeLegibility", "geometricPrecision"
     ] },