optimization_advice.md: Improve vector drawable and pngquant parts

Change-Id: I293c4ea85a90edbfcb28cbcf7bd3f1201550f3ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902912Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713462}

docs/speed/binary_size/optimization_advice.md

@@ -36,17 +36,25 @@
 ## Optimizing Images
 
  * Would a vector image work?
+   * Images that can be described by a series of paths should generally be
+     stored as vectors.
+     * The one exception is if the image will be used pre-Lollipop in a
+       notification or application icon.
    * For images used in native code: [VectorIcon](https://chromium.googlesource.com/chromium/src/+/HEAD/components/vector_icons/README.md).
-   * For Android drawables: [VectorDrawable](https://codereview.chromium.org/2857893003/).
+   * For Android drawables: [VectorDrawable](https://developer.android.com/guide/topics/graphics/vector-drawable-resources).
+     * Convert from `.svg` online using https://inloop.github.io/svg2android/.
      * Optimize vector drawables with [avocado](https://bugs.chromium.org/p/chromium/issues/detail?id=982302).
+     * (Googlers): Find most icons as .svg at [go/icons](https://goto.google.com/icons).
  * Would **lossy** compression make sense (often true for large images)?
    * If so, [use lossy webp](https://codereview.chromium.org/2615243002/).
    * And omit some densities (e.g. add only an xxhdpi version).
- * Would **near-lossless** compression make sense?
-   * This can often reduce size by >50% without a perceptible difference.
-   * [Use pngquant](https://pngquant.org) to try this out (use one of the GUI
-     tools to compare before/after).
- * Are the **lossless** images fully optimized?
+ * For lossless `.png` images, see how few unique colors you can use without a
+   noticeable difference.
+   * This can often reduce an already optimized .png by 33%-50%.
+   * [Use pngquant](https://pngquant.org) to try this out.
+     * Requires trial and error for each number of unique colors.
+     * Use one of the GUI tools linked from the website to do this easily.
+ * Finally - Ensure .png files are fully optimized.
    * Use [tools/resources/optimize-png-files.sh](https://cs.chromium.org/chromium/src/tools/resources/optimize-png-files.sh).
    * There is some [Googler-specific guidance](https://goto.google.com/clank/engineering/best-practices/adding-image-assets) as well.