Run clang-tidy modernize-use-bool-literals on //third_party/pdfium

See the bugs and cxx post for justification and details:
https://groups.google.com/a/chromium.org/forum/#!topic/cxx/RkOHzIK6Tq8

This change was done using clang-tidy as described here:
https://chromium.googlesource.com/chromium/src/+/lkcr/docs/clang_tidy.md

Bug: chromium:778955
Change-Id: Ib037ed749539e4380fb336fc6fa85648fc506118
Reviewed-on: https://pdfium-review.googlesource.com/19990
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/third_party/base/numerics/safe_conversions_impl.h b/third_party/base/numerics/safe_conversions_impl.h
index 2735c1e..4077b28 100644
--- a/third_party/base/numerics/safe_conversions_impl.h
+++ b/third_party/base/numerics/safe_conversions_impl.h
@@ -143,7 +143,7 @@
  public:
   constexpr RangeCheck(bool is_in_lower_bound, bool is_in_upper_bound)
       : is_underflow_(!is_in_lower_bound), is_overflow_(!is_in_upper_bound) {}
-  constexpr RangeCheck() : is_underflow_(0), is_overflow_(0) {}
+  constexpr RangeCheck() : is_underflow_(false), is_overflow_(false) {}
   constexpr bool IsValid() const { return !is_overflow_ && !is_underflow_; }
   constexpr bool IsInvalid() const { return is_overflow_ && is_underflow_; }
   constexpr bool IsOverflow() const { return is_overflow_ && !is_underflow_; }