Delete tautologous pointer CHECKs in BigEndianReader/BigEndianWriter.

The constructors for BigEndianReader and BigEndianWriter currently check
that |ptr_ <= end_|. Given that |end_| was constructed from |ptr_| plus
an unsigned length and that the constructors' inputs must correspond to
a valid buffer, CHECK_LE(ptr_, end_) can never fail.

Bug: 1114803
Change-Id: I9ae6c454e28967b036a8e1343d8e9a4cba166cdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350151Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797029}

base/big_endian.cc

@@ -12,9 +12,7 @@
 namespace base {
 
 BigEndianReader::BigEndianReader(const char* buf, size_t len)
-    : ptr_(buf), end_(ptr_ + len) {
-  CHECK_LE(ptr_, end_);
-}
+    : ptr_(buf), end_(ptr_ + len) {}
 
 bool BigEndianReader::Skip(size_t len) {
   if (len > remaining())
@@ -88,9 +86,7 @@ bool BigEndianReader::ReadU16LengthPrefixed(base::StringPiece* out) {
 }
 
 BigEndianWriter::BigEndianWriter(char* buf, size_t len)
-    : ptr_(buf), end_(ptr_ + len) {
-  CHECK_LE(ptr_, end_);
-}
+    : ptr_(buf), end_(ptr_ + len) {}
 
 bool BigEndianWriter::Skip(size_t len) {
   if (len > remaining())