GCC: Can't use alignas() together with __attribute__()

It's because GCC has problems when mixing the alignas() together with
__attribute__() (that is used to export the symbols). The best
solution is to use ALIGNAS() macro from //base/compiler_specific.h
together with alignof() to have the equal functionality that compiles on
GCC as well as on clang.

This is a regression after https://crrev.com/659933.

Bug: 994581
Change-Id: Ia376866ff9ac57d52d2e907fa325ae5a51893d07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758064Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Tomáš Popela <tomas.popela@gmail.com>
Auto-Submit: Tomáš Popela <tomas.popela@gmail.com>
Cr-Commit-Position: refs/heads/master@{#688417}

third_party/blink/renderer/core/css/css_property_value_set.h

@@ -175,8 +175,8 @@ class CSSLazyPropertyParser
   DISALLOW_COPY_AND_ASSIGN(CSSLazyPropertyParser);
 };
 
-class CORE_EXPORT alignas(Member<const CSSValue>) alignas(
-    CSSPropertyValueMetadata) ImmutableCSSPropertyValueSet
+class CORE_EXPORT ALIGNAS(alignof(Member<const CSSValue>))
+    ALIGNAS(alignof(CSSPropertyValueMetadata)) ImmutableCSSPropertyValueSet
     : public CSSPropertyValueSet {
  public:
   ImmutableCSSPropertyValueSet(const CSSPropertyValue*,