Update asan docs.

Change-Id: I1abf5400e9c3dea53d22e6c0142887d37ca5e5e3
Reviewed-on: https://chromium-review.googlesource.com/1115429
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570855}

docs/linux_running_asan_tests.md

 # Running Chrome tests with AddressSanitizer (asan) and LeakSanitizer (lsan)
 
-It is relatively straightforward to run asan/lsan tests. The only tricky part
-is that some environment variables need to be set.
+Running asan/lsan tests requires changing the build and setting a few
+environment variables.
 
 Changes to args.gn (ie, `out/Release/args.gn`):
 
@@ -10,10 +10,17 @@ is_asan = true
 is_lsan = true
 ```
 
-How to run the test:
+Setting up environment variables and running the test:
 
 ```sh
 $ export ASAN_OPTIONS="symbolize=1 external_symbolizer_path=./third_party/llvm-build/Release+Asserts/bin/llvm-symbolizer detect_leaks=1 detect_odr_violation=0"
 $ export LSAN_OPTIONS=""
 $ out/Release/browser_tests
 ```
+
+Stack traces (such as those emitted by `base::debug::StackTrace().Print()`) may
+not be fully symbolized. The following snippet can symbolize them:
+
+```sh
+$ out/Release/browser_tests 2>&1 | ./tools/valgrind/asan/asan_symbolize.py
+```