supersize: Add "diff" command

tools/binary_size/README.md

 # Tools for analyzing Chrome's binary size
 
 # Super Size
 
 Collect, archive, and analyze Chrome's binary size.
 
 ## "archive"
 
 Collect size information and dump it into a `.size` file. Mainly consists of
 symbol information parsed from a linker .map file.
@@ skipping 15 matching lines @@
     # Non-Googlers:
     gn gen out/Release --args='is_official_build=true symbol_level=1 exclude_unwind_tables=true ffmpeg_branding="Chrome" proprietary_codecs=true'
     ninja -C out/Release -j 1000 chrome
     tools/binary_size/supersize archive chrome.size --elf-file out/Release/chrome -v
 
 ## "html_report"
 
 Creates an interactive size breakdown (by source path) as a stand-alone html
 report.
 
-## Example Usage:
+### Example Usage:
 
     tools/binary_size/supersize html_report chrome.size --report-dir size-report -v
     xdg-open size-report/index.html
 
 ## "console"
 
 Starts a Python interpreter where you can run custom queries.
 
-## Example Usage:
+### Example Usage:
 
-    # Runs a single diff and exits (does not enter interactive mode).
-    tools/binary_size/supersize console without_patch.size with_patch.size --query='Diff(size_info2, size_info1)'
+    # Prints size infomation and exits (does not enter interactive mode).
+    tools/binary_size/supersize console chrome.size --query='Print(size_info)'
 
     # Enters a Python REPL (it will print more guidance).
     tools/binary_size/supersize console chrome.size
 
-## diagnose_apk_bloat.py
+## "diff"
+
+A convenience command equivalent to: `console before.size after.size --query='Print(Diff(size_info1, size_info2))'`
+
+### Example Usage:
+
+    tools/binary_size/supersize diff before.size after.size --all
+
+# diagnose_apk_bloat.py
 
 Determine the cause of binary size bloat for a patch.
 
-### Example Usage:
+## Example Usage:
 
     # Sync, build, and diff for HEAD and HEAD^.
     tools/binary_size/diagnose_apk_bloat.py
 
     # Display detailed usage info (there are many options).
     tools/binary_size/diagnose_apk_bloat.py -h
 
 # Roadmap for Super Size:
 
 Tracked in https://crbug.com/681694
@@ skipping 23 matching lines @@
 1. Integrate with `resource_sizes.py` so that it tracks size of major
    components separately: chrome vs blink vs skia vs v8.
 1. Speed up some steps (like normalizing names) via multiprocessing.
 1. Add dependency graph info, perhaps just on a per-file basis.
 
 # Roadmap for diagnose_apk_bloat.py:
 1. More `diagnose_apk_bloat.py` features:
 
   * Add diffing functionality to see diff stats for two commits.
   * Add --cloud option for using artifacts from perf builders.