Migrate to PrerenderTestHelper::SetUp

A follow up to crrev.com/c/3031862 where I'd added a
SetUp function in addition to SetUpOnMainThread. Although
either could be used, the latter was not necessary. This
change removes SetUpOnMainThread.

Generally, this moves the call to the fixture's SetUp
method as well, but this was not done in a number cases:
 - lite video browser test has ordering constraints due to
   its use of ScopedFeatureList, and
 - blink optimization guide browsertest calls SetUp from
   SetUpOnMainThread for the same reasons as the lite video
   browser test
 - the push messaging browser test has ordering constraints
   due to when the embedded server is created.

I've added comments in both of these cases and a more detailed
comment to PrerenderTestHelper::SetUp to make it clear when
it would ideally be called and reasons why you may make other
choices.

(This CL recreates crrev.com/c/3035682 where I'd mistakenly
cleared the cc list)

Bug: 1230090
Change-Id: I305f7a696cf425ecb4f74a4809d36ddc864ecd7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3061984


Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908673}