Specify handling "defined" operators generated by macro replacement

This is already tested by the dEQP shader preprocessor tests in the
conformance suite.

Five different corner cases of this behavior were tested against Nexus 5,
Nexus 6, Nexus 9 and Nexus Player native GLES3 implementations, as well
as the recent implementation in ANGLE TOT. The existing implementations
largely agree with the spec in this commit. The only exception is Nexus
Player, where in the following kind of case:

 #define CCC 1
 #define defined BBB
 #define AAA defined
 #if AAA CCC
 #endif

"defined" in the replacement list of "AAA" is replaced by "BBB" instead
of immediately handling it as the "defined" operator. In all the other
four implementations, the defined operator is evaluated in this case and
returns "1".

specs/latest/1.0/index.html

@@ -28,7 +28,7 @@
     <!--end-logo-->
 
     <h1>WebGL Specification</h1>
-    <h2 class="no-toc">Editor's Draft 14 September 2015</h2>
+    <h2 class="no-toc">Editor's Draft 29 September 2015</h2>
     <dl>
         <dt>This version:
             <dd>
@@ -3840,6 +3840,28 @@ <h3>GLSL ES Global Variable Initialization</h3>
     content.
 </div>
 
+<h3><a name="PREPROCESSOR_DEFINED_IN_MACRO">GLSL ES Preprocessor "defined" Operator Generated By Macro Replacement</a></h3>
+
+<p>
+    The C++ standard, which the GLSL ES preprocessor specification refers to, has undefined behavior
+    when the <code>defined</code> operator is generated by macro replacement when parsing the
+    controlling expression of an <code>#if</code> or <code>#elif</code> directive. When shader code
+    processed by the WebGL API generates the token <code>defined</code> during macro replacement
+    inside a preprocessor expression, that token must be immediately handled as the
+    <code>defined</code> operator before any further macro replacement.
+</p>
+
+<p>
+    This has no effect on macro expansion outside preprocessor directives that handle the
+    <code>defined</code> operator.
+</p>
+
+<div class="note rationale">
+    This behavior is enforced across GLES implementations by dEQP tests, so shaders created on other
+    platforms may rely on it. In addition, the ported dEQP tests in the WebGL conformance suite are
+    only valid if this behavior is specified.
+</div>
+
 <!-- ======================================================================================================= -->
 
     <h2>References</h2>