Add default PDF dpi setting

Make sure save as PDF sets DPI to a nonzero value. Since we now check it
in Windows, if the job settings were previously invalid it will crash if
the printer does not reset them correctly.

BUG=None

Review-Url: https://codereview.chromium.org/2847003007
Cr-Commit-Position: refs/heads/master@{#468351}

printing/printing_context.cc

@@ -65,6 +65,8 @@ PrintingContext::Result PrintingContext::UsePdfSettings() {
   pdf_settings->SetBoolean(kSettingCollate, true);
   pdf_settings->SetInteger(kSettingCopies, 1);
   pdf_settings->SetInteger(kSettingColor, printing::COLOR);
+  pdf_settings->SetInteger(kSettingDpiHorizontal, kPointsPerInch);
+  pdf_settings->SetInteger(kSettingDpiVertical, kPointsPerInch);
   pdf_settings->SetInteger(kSettingDuplexMode, printing::SIMPLEX);
   pdf_settings->SetBoolean(kSettingLandscape, false);
   pdf_settings->SetString(kSettingDeviceName, "");