Add lowLatency flag to WebGLContextAttributes.

Developers of stylus-based drawing applications (e.g. Google Keep)
have found this hint to be critical in order to be competitive with
native applications. It has been prototyped in Chromium and yields
significant latency improvements there.

This is the WebGL analogue of whatwg/html#4234
and partially addresses whatwg/html#4087 .

specs/latest/1.0/index.html

@@ -744,6 +744,7 @@ <h3><a name="WEBGLCONTEXTATTRIBUTES">WebGLContextAttributes</a></h3>
     GLboolean preserveDrawingBuffer = false;
     WebGLPowerPreference powerPreference = "default";
     GLboolean failIfMajorPerformanceCaveat = false;
+    GLboolean lowLatency = false;
 };</pre>
 
     <h4>Context creation parameters</h4>
@@ -877,6 +878,21 @@ <h4>Context creation parameters</h4>
               set to <code>false</code>, with the knowledge that a reduced-fidelity rendering mode
               should be used to improve performance.
             </dd>
+        <dt> <span class=prop-value>lowLatency</span></dt>
+            <dd>
+              <p>
+                If the value is true, the user agent will attempt to optimize the rendering of the
+                canvas to reduce latency, including potentially rendering from the front buffer.
+                This might desynchronize the canvas's display from the rest of the page and induce
+                tearing on some displays.
+              </p>
+              <div class="note">
+                The <code>lowLatency</code> flag is useful and necessary when implementing certain
+                kinds of applications, such as drawing applications, where the delay between input
+                and rasterization is critical. However, applications should not arbitrarily set
+                the <code>lowLatency</code> flag to true; it should be used only when needed.
+              </div>
+            </dd>
     </dl>
     <div class="example">
     Here is an ECMAScript example which passes a WebGLContextAttributes argument

specs/latest/1.0/webgl.idl

@@ -54,6 +54,7 @@ dictionary WebGLContextAttributes {
     GLboolean preserveDrawingBuffer = false;
     WebGLPowerPreference powerPreference = "default";
     GLboolean failIfMajorPerformanceCaveat = false;
+    GLboolean lowLatency = false;
 };
 
 [Exposed=(Window,Worker)]