Add wrapper script for computing n-grams from SKPs on Cluster Telemetry

tools/lua/ngrams.lua

 -- Generate n-grams of Skia API calls from SKPs.
 
 -- To test this locally, run:
 -- $ GYP_DEFINES="skia_shared_lib=1" make lua_pictures
 -- $ out/Debug/lua_pictures -q -r $SKP_DIR -l tools/lua/ngrams.lua > /tmp/lua-output
 -- $ lua tools/lua/ngrams_aggregate.lua
 
 -- To run on Cluster Telemetry, copy and paste the contents of this file into
 -- the box at https://skia-tree-status.appspot.com/skia-telemetry/lua_script,
 -- and paste the contents of ngrams_aggregate.lua into the "aggregator script"
 -- box on the same page.
 
 -- Change n as desired.
+-- CHANGEME
 local n = 3
+-- CHANGEME
 
 -- This algorithm uses a list-of-lists for each SKP. For API call, append a
 -- list containing just the verb to the master list. Then, backtrack over the
 -- last (n-1) sublists in the master list and append the verb to those
 -- sublists. At the end of execution, the master list contains a sublist for
 -- every verb in the SKP file. Each sublist has length n, with the exception of
 -- the last n-1 sublists, which are discarded in the summarize() function,
 -- which generates counts for each n-gram.
 
 local ngrams = {}
@@ skipping 29 matching lines @@
         counts[ngram] = counts[ngram] + 1
       end
     end
   end
 
   -- Write out code for aggregating.
   for ngram, count in pairs(counts) do
     io.write("if counts['", ngram, "'] == nil then counts['", ngram, "'] = ", count, " else counts['", ngram, "'] = counts['", ngram, "'] + ", count, " end\n")
   end
 end