[memory] Add a test for RX page permission.

Bug: chromium:799573
Change-Id: I467c74b5b26bc6f1e1a35c1104df12b77f072bac
Reviewed-on: https://chromium-review.googlesource.com/923430Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51338}

test/unittests/allocation-unittest.cc

@@ -112,6 +112,7 @@ TEST_F(MemoryAllocationPermissionsTest, DoTest) {
   TestPermissions(PageAllocator::Permission::kNoAccess, false, false);
   TestPermissions(PageAllocator::Permission::kReadWrite, true, true);
   TestPermissions(PageAllocator::Permission::kReadWriteExecute, true, true);
+  TestPermissions(PageAllocator::Permission::kReadExecute, true, false);
 }
 #endif  // V8_OS_POSIX