[conversions] Do not use a noise provider in debug_mode

Fixes flakes in ConversionsBrowsertest caused by a real noise
provider being used on conversiondata.

The ternary used to allocate a noise provider is backwards, and should
use a null NoiseProvider in debug mode.

Bug: 1082236,1082168,1082134
Change-Id: Ibaa23ed16aaed1a80aa90f9e25a9ba8fd211f4bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198367
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768354}