Revert of Temporarily disable double fields unboxing. (patchset #1 id:1 of...

Revert of Temporarily disable double fields unboxing. (patchset #1 id:1 of https://codereview.chromium.org/928733003/)

Reason for revert:
Preparing to land a fix.

Original issue's description:
> Temporarily disable double fields unboxing.
>
> Committed: https://crrev.com/209cf09ac9e36c1a24cdfa918bc579a4671c6842
> Cr-Commit-Position: refs/heads/master@{#26727}

TBR=jkummerow@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/960173002

Cr-Commit-Position: refs/heads/master@{#26876}

src/globals.h

@@ -89,7 +89,7 @@ namespace internal {
 
 // Determine whether double field unboxing feature is enabled.
 #if V8_TARGET_ARCH_64_BIT
-#define V8_DOUBLE_FIELDS_UNBOXING 0
+#define V8_DOUBLE_FIELDS_UNBOXING 1
 #else
 #define V8_DOUBLE_FIELDS_UNBOXING 0
 #endif