DM: display current memory usage (instead of peak) when available.

Seems strictly more useful.

This implements Mac and Windows, which seemed easy.  Don't know how to do this on Linux yet.

BUG=skia:

CQ_EXTRA_TRYBOTS=client.skia:Test-Mac10.9-MacMini6.2-HD4000-x86_64-Debug-Trybot

NOTREECHECKS=true
TBR=halcanary@google.com

Review URL: https://codereview.chromium.org/990723002
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index d66b308..920979c 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -476,7 +476,7 @@
         if (FLAGS_mpd) {
             fUseMPDs.push_back() = true;
         }
-        
+
         // Prepare the images for decoding
         for (int i = 0; i < FLAGS_images.count(); i++) {
             const char* flag = FLAGS_images[i];
@@ -492,7 +492,7 @@
                 fImages.push_back() = flag;
             }
         }
-        
+
         // Choose the candidate color types for image decoding
         const SkColorType colorTypes[] =
             { kN32_SkColorType, kRGB_565_SkColorType, kAlpha_8_SkColorType };
@@ -842,7 +842,7 @@
                     config = ""; // Only print the config if we run the same bench on more than one.
                 }
                 SkDebugf("%4dM\t%s\t%s\n"
-                         , sk_tools::getMaxResidentSetSizeMB()
+                         , sk_tools::getBestResidentSetSizeMB()
                          , bench->getUniqueName()
                          , config);
             } else if (FLAGS_verbose) {
@@ -858,7 +858,7 @@
             } else {
                 const double stddev_percent = 100 * sqrt(stats.var) / stats.mean;
                 SkDebugf("%4dM\t%d\t%s\t%s\t%s\t%s\t%.0f%%\t%s\t%s\t%s\n"
-                        , sk_tools::getMaxResidentSetSizeMB()
+                        , sk_tools::getBestResidentSetSizeMB()
                         , loops
                         , HUMANIZE(stats.min)
                         , HUMANIZE(stats.median)