Set symbol level to 1 on ToTLinuxThinLTO builder.

A symbol level of 2 is causing out of disk space errors on the bot.
While it's probably worth building everything in order to catch ThinLTO
bugs that may affect developers, there probably isn't as much value
in building all of it at symbol level 2.

Bug: 807402
Change-Id: I40930a0c14bb1fbee2b324f4463a666497dcec61
Reviewed-on: https://chromium-review.googlesource.com/907760
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535244}