Split pdfium::size() off to cxx17_backports.h.

Follow Chromium's efforts to split stl_util.h. Do some IWYU for
pdfium::size() as well.

Change-Id: Ic50f5b1e7f0da95aa93742a591792628cd4525ae
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/82175
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/fpdf_text_embeddertest.cpp b/fpdfsdk/fpdf_text_embeddertest.cpp
index 73754ec..4b653bd 100644
--- a/fpdfsdk/fpdf_text_embeddertest.cpp
+++ b/fpdfsdk/fpdf_text_embeddertest.cpp
@@ -16,7 +16,7 @@
 #include "testing/embedder_test.h"
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
-#include "third_party/base/stl_util.h"
+#include "third_party/base/cxx17_backports.h"
 
 namespace {