Enable full symbols in Windows ChromeDriver build

Change ChromeDriver build bot setting for Windows, to generate
full symbols.

Change-Id: I31ea9abd5ff619f12f15da8ef46ad01590f162a3
Reviewed-on: https://chromium-review.googlesource.com/994735Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547984}

tools/mb/mb_config.pyl

@@ -85,7 +85,7 @@
     },
 
     'chromium.chromedriver': {
-      'Win7': 'release_bot_x86_minimal_symbols',
+      'Win7': 'release_bot_x86',
       'Mac 10.6': 'release_bot',
       'Linux': 'release_bot',
       'Linux32': 'release_bot_x86',