Remove use of AS class constant which is syntax error in PHP<7

Author: westonruter

includes/class-amp-theme-support.php

@@ -1755,7 +1755,7 @@ public static function ensure_required_markup( Document $dom, $script_handles =
 			Tag::LINK,
 			[
 				Attribute::REL         => Attribute::REL_PRELOAD,
-				Attribute::AS          => Tag::SCRIPT,
+				'as'                   => Tag::SCRIPT,
 				Attribute::HREF        => $runtime_src,
 				Attribute::CROSSORIGIN => Attribute::CROSSORIGIN_ANONYMOUS,
 			]
@@ -1775,7 +1775,7 @@ public static function ensure_required_markup( Document $dom, $script_handles =
 				Tag::LINK,
 				[
 					Attribute::REL         => Attribute::REL_PRELOAD,
-					Attribute::AS          => Tag::SCRIPT,
+					'as'                   => Tag::SCRIPT,
 					Attribute::HREF        => $amp_scripts[ $script_handle ]->getAttribute( Attribute::SRC ),
 					Attribute::CROSSORIGIN => Attribute::CROSSORIGIN_ANONYMOUS,
 				]

lib/common/src/Attribute.php

@@ -27,7 +27,6 @@ interface Attribute
     const AMP_RUNTIME           = 'amp-runtime';
     const AMP_SCRIPT_SRC        = 'amp-script-src';
     const ARIA_HIDDEN           = 'aria-hidden';
-    const AS                    = 'as';
     const ASYNC                 = 'async';
     const AUTOPLAY              = 'autoplay';
     const CHARSET               = 'charset';