Revert "[page_allocator] Don't allow read/write/execute permissions."

This reverts commit 39dfc337.

Reason for revert: Switching V8 to use page_allocator is blocked on removal of RWX permissions on that side. Reverting this temporarily so we can switch V8 to page_allocator.

Original change's description:
> [page_allocator] Don't allow read/write/execute permissions.
> 
> - V8 is moving to a read/write read/execute system. We won't need
>   read/write/execute and should remove it from Chrome now.
> 
> Bug: chromium:756050
> Change-Id: Icc8a3ec334535c460e74a413cb265de2872db9f5
> Reviewed-on: https://chromium-review.googlesource.com/822514
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#523663}

TBR=palmer@chromium.org,bbudge@chromium.org,haraken@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:756050
Change-Id: I3fd2b0172822af0ce7e5cf003c53f0b6f69305e8
Reviewed-on: https://chromium-review.googlesource.com/893609Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533130}

base/allocator/partition_allocator/page_allocator.cc

@@ -47,6 +47,8 @@ int GetAccessFlags(PageAccessibilityConfiguration page_accessibility) {
       return PROT_READ | PROT_WRITE;
     case PageReadExecute:
       return PROT_READ | PROT_EXEC;
+    case PageReadWriteExecute:
+      return PROT_READ | PROT_WRITE | PROT_EXEC;
     default:
       NOTREACHED();
       FALLTHROUGH;
@@ -73,6 +75,8 @@ int GetAccessFlags(PageAccessibilityConfiguration page_accessibility) {
       return PAGE_READWRITE;
     case PageReadExecute:
       return PAGE_EXECUTE_READ;
+    case PageReadWriteExecute:
+      return PAGE_EXECUTE_READWRITE;
     default:
       NOTREACHED();
     // Fall through.

base/allocator/partition_allocator/page_allocator.h

@@ -43,6 +43,9 @@ enum PageAccessibilityConfiguration {
   PageInaccessible,
   PageReadWrite,
   PageReadExecute,
+  // This flag is deprecated and will go away soon.
+  // TODO(bbudge) Remove this as soon as V8 doesn't need RWX pages.
+  PageReadWriteExecute,
 };
 
 // Allocate one or more pages.