Update memory-infra doc with descriptions for displayed metrics

docs/memory-infra/README.md

 # MemoryInfra
 
 MemoryInfra is a timeline-based profiling system integrated in chrome://tracing.
 It aims at creating Chrome-scale memory measurement tooling so that on any
 Chrome in the world --- desktop, mobile, Chrome OS or any other --- with the
 click of a button you can understand where memory is being used in your system.
 
 [TOC]
 
 ## Getting Started
@@ skipping 40 matching lines @@
 [m-blue]:            https://storage.googleapis.com/chromium-docs.appspot.com/b60f342e38ff3a3767bbe4c8640d96a2d8bc864b
 [partalloc-details]: https://storage.googleapis.com/chromium-docs.appspot.com/02eade61d57c83f8ef8227965513456555fc3324
 [m-purple]:          https://storage.googleapis.com/chromium-docs.appspot.com/d7bdf4d16204c293688be2e5a0bcb2bf463dbbc3
 [mi-ui-doc]:         https://docs.google.com/document/d/1b5BSBEd1oB-3zj_CBAQWiQZ0cmI0HmjmXG-5iNveLqw/edit
 
 ## Columns
 
 **Columns in blue** reflect the amount of actual physical memory used by the
 process. This is what exerts memory pressure on the system.
 
- * **Total Resident**: (TODO: document this).
- * **Peak Total Resident**: (TODO: document this).
- * **PSS**: (TODO: document this).
- * **Private Dirty**: (TODO: document this).
- * **Swapped**: (TODO: document this).
+ * **Total Resident**: The current working set size of the process, excluding
+   the memory overhead of tracing. On Linux, this returns the resident set size.
+ * **Peak Total Resident**: The overall peak working set size of the process on
+   supported platforms. On Linux kernel versions >= 4.0 the peak usage between
+   two memory dumps is shown.
+ * **PSS**: POSIX only. The process's proportional share of total resident size.
+ * **Private Dirty**: The total size of dirty pages which are not used by any
+   other process.
+ * **Swapped**: The total size of anonymous memory used by process, which is
+   swapped out of RAM.
 
 **Columns in black** reflect a best estimation of the the amount of physical
 memory used by various subsystems of Chrome.
 
  * **Blink GC**: Memory used by [Oilpan][oilpan].
  * **CC**: Memory used by the compositor.
    See [cc/memory][cc-memory] for the full details.
- * **Discardable**: (TODO: document this).
- * **Font Caches**: (TODO: document this).
+ * **Discardable**: Total [discardable][discardable] memory used by the process
+   from various components like Skia caches and Web caches.
+ * **Font Caches**: Size of cache that stores Font shapes and platform Fonts.
  * **GPU** and **GPU Memory Buffer**: GPU memory and RAM used for GPU purposes.
    See [GPU Memory Tracing][gpu-memory].
- * **LevelDB**: (TODO: document this).
+ * **LevelDB**: Memory used for LeveldbValueStore(s), IndexedDB databases and
+   ProtoDatabase(s).
  * **Malloc**: Memory allocated by calls to `malloc`, or `new` for most
      non-Blink objects.
  * **PartitionAlloc**: Memory allocated via [PartitionAlloc][partalloc].
    Blink objects that are not managed by Oilpan are allocated with
    PartitionAlloc.
- * **Skia**: (TODO: document this).
- * **SQLite**: (TODO: document this).
- * **V8**: (TODO: document this).
- * **Web Cache**: (TODO: document this).
+ * **Skia**: Memory used by all resources used by the Skia rendering system.
+ * **SQLite**: Memory used for all sqlite databases.
+ * **Sync**: Memory used by Chrome Sync when signed in.
+ * **UI**: Android only. Memory used by Android java bitmaps for the UI.
+ * **V8**: Memory used by V8 Javascript engine.
+ * **Web Cache**: Memory used by resources downloaded from the Web, like images
+   and scipts.
 
 The **tracing column in gray** reports memory that is used to collect all of the
 above information. This memory would not be used if tracing were not enabled,
 and it is discounted from malloc and the blue columns.
 
 <!-- TODO(primiano): Improve this. https://crbug.com/??? -->
 
 [oilpan]:     /third_party/WebKit/Source/platform/heap/BlinkGCDesign.md
+[discardable]:base/memory/discardable_memory.h
 [cc-memory]:  probe-cc.md
 [gpu-memory]: probe-gpu.md
 [partalloc]:  /third_party/WebKit/Source/wtf/allocator/PartitionAlloc.md
 
 ## Related Pages
 
  * [Adding MemoryInfra Tracing to a Component](adding_memory_infra_tracing.md)
  * [GPU Memory Tracing](probe-gpu.md)
  * [Heap Profiler Internals](heap_profiler_internals.md)
  * [Heap Profiling with MemoryInfra](heap_profiler.md)
@@ skipping 57 matching lines @@
 
 Chrome-side design docs:
 
 <iframe width="100%" height="300px" src="https://docs.google.com/a/google.com/embeddedfolderview?id=0B3KuDeqD-lVJfndSa2dleUQtMnZDeWpPZk1JV0QtbVM5STkwWms4YThzQ0pGTmU1QU9kNVk">
 </iframe>
 
 Catapult-side design docs:
 
 <iframe width="100%" height="300px" src="https://docs.google.com/a/google.com/embeddedfolderview?id=0B3KuDeqD-lVJfm10bXd5YmRNWUpKOElOWS0xdU1tMmV1S3F4aHo0ZDJLTmtGRy1qVnQtVWM">
 </iframe>