CodeHealth: Remove use of Value::GetAsDouble

This change removes the use of Value::GetAsDouble
and replaces with Value::is_double(), Value::GetDouble(),
Value::GetIfDouble().

net/base/backoff_entry_serializer_unittest.cc

Bug: 1187006
Change-Id: Ic1dd4deeef240182772152a2ca579529242c21fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3011124


Auto-Submit: Maksim Sadym <sadym@chromium.org>
Commit-Queue: Victor Vasiliev <vasilvv@chromium.org>
Reviewed-by: Victor Vasiliev <vasilvv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899190}