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/893843004/)

Reason for revert:
Experiment is over, turning it on back.

Original issue's description:
> Temporarily disable double fields unboxing.
>
> Committed: https://crrev.com/51c59ede18831d1c1a1d70fa9fa456f946ac486b
> Cr-Commit-Position: refs/heads/master@{#26492}

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

Review URL: https://codereview.chromium.org/912673003

Cr-Commit-Position: refs/heads/master@{#26550}

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