Document some skia perf interesting queries.

Preview here: http://skiadocs.com:8000/dev/tools/skiaperf?cl=839303002

BUG=skia:

Review URL: https://codereview.chromium.org/839303002

site/dev/index.md

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+Developer Documentation
+=======================
+
+If you want to make changes to the Skia code, this is the place for you.

site/dev/tools/index.md

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+Tools
+=====
+
+Developer tools for accessing the health of Skia, such as
+`nanobench`, `dm`, and [Skia Perf](https://skiaperf.com).
+

site/dev/tools/skiaperf.md

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+Skia Perf
+=========
+
+[Skia Perf](https://skiaperf.com) is a web based interface for exploring
+performance data produced by `nanobench` and the code size bot. The data
+includes:
+
+  * The nanobench test times in ms.
+  * Total memory consumed during a nanobench run.
+  * Code size for various symbol types in bytes.
+
+All of the data can be plotted and also can be [analyzed using k-means
+clustering](https://skiaperf.com/clusters/).
+
+Calculations
+------------
+
+Skia Perf has the ability to perform calculations over the test data
+allowing you to build up interesting queries.
+
+For example, this query displays the [total code size of the library over time](https://skiaperf.com/#1877):
+
+    sum(fill(filter("config=memory&sub_result=bytes")))
+
+This query displays [the ratio of playback time in ms to the number of ops for desk\_wowwiki.skp](https://skiaperf.com/#1876):
+
+    ratio(
+      ave(fill(filter("name=desk_wowwiki.skp&sub_result=min_ms"))),
+      ave(fill(filter("name=desk_wowwiki.skp&sub_result=ops")))
+    )
+
+You can also use the data to answer questions like [how many tests were run per commit](https://skiaperf.com/#1878).
+
+    count(filter(""))
+
+See Skia Perf for the [full list of functions available](https://skiaperf.com/help).