Revert "Add range checks for `StringPiece`."

This reverts commit c8bccdf1.

Reason for revert: This change has r540894 as dependency. However, r540894 was reverted in r541006, just before this patch landed. Since this currently causes failures on Mac due to crashpad errors, I am reverting this change for now.

Original change's description:
> Add range checks for `StringPiece`.
> 
> Range check `operator[]`, `front`, `back`, and  `remove_{pre,suf}fix`.
> 
> Bug: 817982,818376,818844
> Change-Id: I3a0b560af273c9b04237a18a1bc0b56283d3d824
> Reviewed-on: https://chromium-review.googlesource.com/945049
> Commit-Queue: Chris Palmer <palmer@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#541012}

TBR=palmer@chromium.org,dcheng@chromium.org,mark@chromium.org

Change-Id: I7d5c2ca050869aff8dca4ba6b724761172aa1d83
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 817982, 818376, 818844
Reviewed-on: https://chromium-review.googlesource.com/950822Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541060}

base/strings/string_piece.h

@@ -219,31 +219,16 @@ template <typename STRING_TYPE> class BasicStringPiece {
     length_ = str ? STRING_TYPE::traits_type::length(str) : 0;
   }
 
-  constexpr value_type operator[](size_type i) const {
-    CHECK(i < length_);
-    return ptr_[i];
-  }
-
-  value_type front() const {
-    CHECK_NE(0UL, length_);
-    return ptr_[0];
-  }
-
-  value_type back() const {
-    CHECK_NE(0UL, length_);
-    return ptr_[length_ - 1];
-  }
+  constexpr value_type operator[](size_type i) const { return ptr_[i]; }
+  value_type front() const { return ptr_[0]; }
+  value_type back() const { return ptr_[length_ - 1]; }
 
   constexpr void remove_prefix(size_type n) {
-    CHECK(n <= length_);
     ptr_ += n;
     length_ -= n;
   }
 
-  constexpr void remove_suffix(size_type n) {
-    CHECK(n <= length_);
-    length_ -= n;
-  }
+  constexpr void remove_suffix(size_type n) { length_ -= n; }
 
   int compare(const BasicStringPiece<STRING_TYPE>& x) const {
     int r = wordmemcmp(