Enable image decoding attribute.

This patch enables img.decoding attribute.
I2S: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/MbXp16hQclY

R=chrishtr@chromium.org

Change-Id: I4b4387d1582b72787687bdc68dd96ff0f3b6c134
Reviewed-on: https://chromium-review.googlesource.com/828273Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524307}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -582,6 +582,7 @@ html element img
     property crossOrigin
     property currentSrc
     property decode
+    property decoding
     property height
     property hspace
     property isMap
@@ -1735,6 +1736,7 @@ svg element g
     property transform
 svg element image
     property decode
+    property decoding
     property farthestViewportElement
     property getBBox
     property getCTM

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -2205,6 +2205,7 @@ interface HTMLImageElement : HTMLElement
     getter complete
     getter crossOrigin
     getter currentSrc
+    getter decoding
     getter height
     getter hspace
     getter isMap
@@ -2228,6 +2229,7 @@ interface HTMLImageElement : HTMLElement
     setter alt
     setter border
     setter crossOrigin
+    setter decoding
     setter height
     setter hspace
     setter isMap
@@ -3205,6 +3207,7 @@ interface Image
     getter complete
     getter crossOrigin
     getter currentSrc
+    getter decoding
     getter height
     getter hspace
     getter isMap
@@ -3228,6 +3231,7 @@ interface Image
     setter alt
     setter border
     setter crossOrigin
+    setter decoding
     setter height
     setter hspace
     setter isMap
@@ -5178,6 +5182,7 @@ interface SVGGraphicsElement : SVGElement
     method getScreenCTM
 interface SVGImageElement : SVGGraphicsElement
     attribute @@toStringTag
+    getter decoding
     getter height
     getter href
     getter preserveAspectRatio
@@ -5186,6 +5191,7 @@ interface SVGImageElement : SVGGraphicsElement
     getter y
     method constructor
     method decode
+    setter decoding
 interface SVGLength
     attribute @@toStringTag
     attribute SVG_LENGTHTYPE_CM

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -484,7 +484,7 @@
     },
     {
       name: "ImageDecodingAttribute",
-      status: "test",
+      status: "stable",
     },
     {
       name: "ImageOrientation",