Fix clamp() ambiguity for C++17 builds.

Although PDFium's GN-based build system uses C++14, other build systems
may build PDFium in C++17 mode. In that situation, calling clamp()
without a namespace will result in a compiler error due to ambiguities
between pdfium::clamp() and std::clamp().

Bug: chromium:1231569
Change-Id: I643e8fa20fdb40a85e191b79d961e9239190ec63
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/83794
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

third_party/base/cxx17_backports.h

diff --git a/third_party/base/cxx17_backports.h b/third_party/base/cxx17_backports.h
index 0bc781b..9d6a966 100644
--- a/third_party/base/cxx17_backports.h
+++ b/third_party/base/cxx17_backports.h
@@ -38,7 +38,7 @@
 
 template <typename T>
 constexpr const T& clamp(const T& v, const T& lo, const T& hi) {
-  return clamp(v, lo, hi, std::less<T>{});
+  return pdfium::clamp(v, lo, hi, std::less<T>{});
 }
 
 }  // namespace pdfium