Update browser testing documentation.

tools/testing/browser_README.txt

 Overview:
  These are the instructions to run a wide variety of browser tests using 
  test.dart or dart/tools/testing/perf_testing/run_perf_tests.py. Currently 
  the results of run_perf_tests are uploaded to
  https://dartperf.googleplex.com/. 
  
 ========= General Browser Setup ==========
 
 See instructions on:
 https://code.google.com/p/dart/wiki/BrowserTestSetup
 
 ========= Proceed further only if you also want to run performance tests.======
 
-1) Download appengine for Python and place it in third_party (http://code.google.com/appengine/downloads.html#Google_App_Engine_SDK_for_Python):
-  "dart/third_party/appengine-python/"
-
-2) Install matplotlib http://matplotlib.sourceforge.net/
-
-3) Pull down benchmarks from internal repo (Google only): 
+1) Pull down benchmarks from internal repo (Google only): 
    http://chromegw.corp.google.com/viewvc/dash/trunk/internal/browserBenchmarks/README.txt?view=markup
 
-4) Create a directory in called appengine-python in third_party. Download and 
-   install App Engine in the directory you just created. 
-   http://code.google.com/appengine/downloads.html
+2) Create a directory in called appengine-python in third_party. Download the 
+   Linux/Other Platforms .zip file, and place the contents in the directory 
+   you just created. 
+   http://code.google.com/appengine/downloads.html#Google_App_Engine_SDK_for_Python
 
-5) Run the tests! While standing in dart/tools/testing/perf_testing, run 
+3) Run the tests! While standing in dart/tools/testing/perf_testing, run 
    $> python run_perf_tests.py --forever --verbose 
    to run all the tests (browser performance, language correctness in the
    browser, command line performance, and self-hosted compile time and compiled
    code size). 
 
    You can run individual tests by adding the particular option (such as 
    --language) when running create_graph.py. Type "create_graph.py -h" for a 
    full list of the options.