add customized printing setting for headless

content/browser/devtools/protocol/page_handler.h

Index: content/browser/devtools/protocol/page_handler.h
diff --git a/content/browser/devtools/protocol/page_handler.h b/content/browser/devtools/protocol/page_handler.h
index faace0adfaf25306aba30490bb643e2128e4f982..62d5ebbf23d258a488ebd7875cb6841a2042a165 100644
--- a/content/browser/devtools/protocol/page_handler.h
+++ b/content/browser/devtools/protocol/page_handler.h
@@ -77,7 +77,19 @@ class PageHandler : public DevToolsDomainHandler,
       Maybe<int> quality,
       Maybe<bool> from_surface,
       std::unique_ptr<CaptureScreenshotCallback> callback) override;
-  void PrintToPDF(std::unique_ptr<PrintToPDFCallback> callback) override;
+  void PrintToPDF(Maybe<int> dpi,

[Lei Zhang, 2017/04/20 08:35:58]

Not sure how well a DPI setting will be supported. Have you tried it out?

[jzfeng, 2017/04/27 06:56:05]

Sadly setting different dpi's yield the same pdf file. 
The reason is, the page size is computed from inch to dpi in headless print
manager, and then in print_web_view_helper.cc, dpi is only used to convert the
page size back to inch.

Hence, I removed it and always use kPointsPerInch as dpi on all platforms.
Please let me know if there are better options.

[Eric Seckler, 2017/04/27 08:56:59]

Lei: Would there be a way to specify a DPI setting that affects the resolution
of images in the PDF etc? Or is that not supported by the printing stack?

[Lei Zhang, 2017/04/27 22:17:46]

Since we don't expose such a setting, I'd say it's certainly not well supported.
If you really want to know, we can start at the bottom layer and ask the Skia
folks if SkPDF supports a DPI setting.

[Eric Seckler, 2017/04/28 08:13:06]

Let's go without it for now then. I believe someone asked on the headless bug
for an option to set DPI, so we might want to query Skia folks about this at
some point.

+                  Maybe<bool> landscape,
+                  Maybe<bool> display_header_footer,
+                  Maybe<bool> print_backgrounds,
+                  Maybe<double> scale,
+                  Maybe<String> paper_type,
+                  Maybe<String> margin_type,
+                  Maybe<double> margin_top,
+                  Maybe<double> margin_bottom,
+                  Maybe<double> margin_left,
+                  Maybe<double> margin_right,
+                  Maybe<String> page_ranges,
+                  std::unique_ptr<PrintToPDFCallback> callback) override;
   Response StartScreencast(Maybe<std::string> format,
                            Maybe<int> quality,
                            Maybe<int> max_width,