Ensure that `PartitionAllocGenericFlags` callers are only using valid flags.

Bug: 680657
Change-Id: I2d879f5b28c392a083fbd867a3eb05568d19dc67
Reviewed-on: https://chromium-review.googlesource.com/1164252Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581049}

base/allocator/partition_allocator/partition_alloc.h

@@ -344,6 +344,8 @@ ALWAYS_INLINE void* PartitionAllocGenericFlags(PartitionRootGeneric* root,
                                                int flags,
                                                size_t size,
                                                const char* type_name) {
+  DCHECK_LT(flags, PartitionAllocLastFlag << 1);
+
 #if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
   void* result = malloc(size);
   CHECK(result || flags & PartitionAllocReturnNull);

base/allocator/partition_allocator/partition_alloc_constants.h

@@ -154,6 +154,8 @@ static const size_t kReasonableSizeOfUnusedPages = 1024 * 1024 * 1024;  // 1GB
 // Flags for PartitionAllocGenericFlags.
 enum PartitionAllocFlags {
   PartitionAllocReturnNull = 1 << 0,
+
+  PartitionAllocLastFlag = PartitionAllocReturnNull
 };
 
 }  // namespace base