Add pre_log option to nanobench; make this option default true in nanobench and dm.

tools/dm_flags.json

Index: tools/dm_flags.json
diff --git a/tools/dm_flags.json b/tools/dm_flags.json
index 6fe4d3c571cc67e8442d0aa8238d824cc7577433..bf88638fc732c236dd80df78e82b606f146fbe37 100644
--- a/tools/dm_flags.json
+++ b/tools/dm_flags.json
@@ -1,5 +1,6 @@
 {
   "Pretend-iOS-Bot": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1", 
@@ -375,6 +376,7 @@
     "image-cacherator-from-ctable"
   ], 
   "Test-Android-GCC-AndroidOne-GPU-Mali400MP2-Arm7-Release": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -723,6 +725,7 @@
     "~WritePixels"
   ], 
   "Test-Android-GCC-GalaxyS3-GPU-Mali400-Arm7-Debug": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -1074,6 +1077,7 @@
     "~WritePixels"
   ], 
   "Test-Android-GCC-GalaxyS4-GPU-SGX544-Arm7-Release": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -1427,6 +1431,7 @@
     "~ReadPixels"
   ], 
   "Test-Android-GCC-Nexus7-GPU-Tegra3-Arm7-Release": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -1772,6 +1777,7 @@
     "image-cacherator-from-ctable"
   ], 
   "Test-Android-GCC-Nexus9-GPU-TegraK1-Arm64-Debug": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -2119,6 +2125,7 @@
     "image-cacherator-from-ctable"
   ], 
   "Test-Android-GCC-NexusPlayer-CPU-SSSE3-x86-Release": [
+    "--pre_log", 
     "--config", 
     "565", 
     "8888", 
@@ -2476,6 +2483,7 @@
     "~ResourceCache"
   ], 
   "Test-Mac10.8-Clang-MacMini4.1-CPU-SSE4-x86_64-Release": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1", 
@@ -2848,6 +2856,7 @@
     "image-cacherator-from-ctable"
   ], 
   "Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1", 
@@ -3221,6 +3230,7 @@
     "image-cacherator-from-ctable"
   ], 
   "Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Valgrind": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1", 
@@ -3616,6 +3626,7 @@
     "~Threaded"
   ], 
   "Test-Ubuntu-GCC-ShuttleA-GPU-GTX550Ti-x86_64-Release-Valgrind": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1", 
@@ -4006,6 +4017,7 @@
     "~Threaded"
   ], 
   "Test-Win7-MSVC-ShuttleA-GPU-HD2000-x86-Debug-ANGLE": [
+    "--pre_log", 
     "--matrix", 
     "0", 
     "1",