doc: add instructions to capture a `.skp` from chromium

site/user/tips.md

 Tips & FAQ
 ==========
 
 Tips and Tricks
 ---------------
 
 ### Bitmap Subsetting
 
 Taking a subset of a bitmap is effectively free - no pixels are copied or
 memory is allocated. This allows Skia to offer an API that typically operates
 on entire bitmaps; clients who want to operate on a subset of a bitmap can use
 the following pattern, here being used to magnify a portion of an image with
 drawBitmapNine():
 
     SkBitmap subset;
     bitmap.extractSubset(&subset, rect);
     canvas->drawBitmapNine(subset, ...);
 
+### Capturing a `.skp` file on a web page in Chromium.
+
+1.  Launch Chrome or Chromium with `--no-sandbox --enable-gpu-benchmarking`
+2.  Open the JS console (ctrl-shift-J)
+3.  Execute: `chrome.gpuBenchmarking.printToSkPicture('/tmp')`
+    This returns "undefined" on success.
+
+Open the resulting file in the Skia Debugger:
+
+    bin/sync-and-gyp
+    ninja -C out/Release debugger
+    out/Release/debugger /tmp/layer_0.skp &
+
+Or use `dm` to rasterize it.
+
+    bin/sync-and-gyp
+    ninja -C out/Release dm
+    out/Release/dm --src skp --skps /tmp/layer_0.skp -w /tmp \
+        --config 8888 gpu pdf --verbose
+    ls -l /tmp/*/skp/layer_0.skp.*
+
 FAQ
 ---
 
 ### Does Skia support HW acceleration?
 
 
 There are two ways Skia can take advantage of HW.
 
 1. Subclass SkCanvas
 
@@ skipping 10 matching lines @@
 ### Does Skia support Font hinting?
 
 Skia has a built-in font cache, but it does not know how to actual render font
 files like TrueType? into its cache. For that it relies on the platform to
 supply an instance of SkScalerContext?. This is Skia's abstract interface for
 communicating with a font scaler engine. In src/ports you can see support
 files for FreeType?, Mac OS X, and Windows GDI font engines. Other font
 engines can easily be supported in a like manner.