Exclude BigEndianWriter::ptr_ from rewrite_raw_ptr_fields

BigEndianWriter::ptr_ may point past the end of an allocation.

Bug: 1073933, 1164636
Change-Id: Ibaac7e81e08984e8e7519f10dbb8106a70f31039
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639054Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845082}

tools/clang/rewrite_raw_ptr_fields/manual-fields-to-ignore.txt

@@ -131,6 +131,7 @@ KeyedServiceBaseFactory::service_name_ # used in decltype
 # Populated manually - this field points past the end of an allocation;
 # therefore, it can refer to an invalid allocation slot
 base::BigEndianWriter::end_
+base::BigEndianWriter::ptr_
 
 # ELEMENT() treats the CheckedPtr as a void*, and so when a pointer is written
 # AddRef() won't be called, causing AddRef/Deref mismatch.