Fixup coverage build

The original coverage CL made the unittests build correclty with coverage but
the code in samples/ builds differently. This CL fixes coverage for pdfium_diff
and pdfium_test code.

Review-Url: https://codereview.chromium.org/2649403003
diff --git a/samples/BUILD.gn b/samples/BUILD.gn
index b4baf2b..11bc9ba 100644
--- a/samples/BUILD.gn
+++ b/samples/BUILD.gn
@@ -14,6 +14,8 @@
 }
 
 config("pdfium_samples_config") {
+  cflags = []
+  ldflags = []
   defines = [
     "PNG_PREFIX",
     "PNG_USE_READ_MACROS",
@@ -28,6 +30,14 @@
   if (pdf_use_skia) {
     defines += [ "PDF_ENABLE_SKIA" ]
   }
+  if (use_coverage && is_clang) {
+    cflags += [
+      "--coverage",
+      "-g",
+      "-O0",
+    ]
+    ldflags += [ "--coverage" ]
+  }
 }
 
 executable("pdfium_test") {