Allow marking pages read-only

Add PageRead to PageAccessibilityConfiguration so pages can be marked
read-only.

Matching CL in V8:
https://chromium-review.googlesource.com/c/v8/v8/+/1006756

Bug: v8:7464
Change-Id: I9acdd9d5c00db9b89b1a2103be83fa2dde257b37
Reviewed-on: https://chromium-review.googlesource.com/1013564Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553513}

base/allocator/partition_allocator/page_allocator.h

@@ -41,6 +41,7 @@ static const size_t kSystemPageBaseMask = ~kSystemPageOffsetMask;
 
 enum PageAccessibilityConfiguration {
   PageInaccessible,
+  PageRead,
   PageReadWrite,
   PageReadExecute,
   // This flag is deprecated and will go away soon.

base/allocator/partition_allocator/page_allocator_internals_posix.h

@@ -29,6 +29,8 @@ std::atomic<int32_t> s_allocPageErrorCode{0};
 
 int GetAccessFlags(PageAccessibilityConfiguration accessibility) {
   switch (accessibility) {
+    case PageRead:
+      return PROT_READ;
     case PageReadWrite:
       return PROT_READ | PROT_WRITE;
     case PageReadExecute:

base/allocator/partition_allocator/page_allocator_internals_win.h

@@ -15,6 +15,8 @@ std::atomic<int32_t> s_allocPageErrorCode{ERROR_SUCCESS};
 
 int GetAccessFlags(PageAccessibilityConfiguration accessibility) {
   switch (accessibility) {
+    case PageRead:
+      return PAGE_READONLY;
     case PageReadWrite:
       return PAGE_READWRITE;
     case PageReadExecute:

gin/v8_platform.cc

@@ -159,6 +159,8 @@ base::LazyInstance<TimeClamper>::Leaky g_time_clamper =
 base::PageAccessibilityConfiguration GetPageConfig(
     v8::PageAllocator::Permission permission) {
   switch (permission) {
+    case v8::PageAllocator::Permission::kRead:
+      return base::PageRead;
     case v8::PageAllocator::Permission::kReadWrite:
       return base::PageReadWrite;
     case v8::PageAllocator::Permission::kReadWriteExecute: