Reland "[getUserMedia] Fix selection of AEC2 over AEC3 when override is not present."

This is a reland of 5ff8ab34825598b7a525cc0631a866320f9039d1

Original change's description:
> [getUserMedia] Fix selection of AEC2 over AEC3 when override is not present.
> 
> During the rewrite of select settings for getUserMedia, a check was
> introduced that verified whether the override_aec3 optional value was
> not base::nullptr. However, this is incorrect as this was not returning the
> default selection for AEC3 but rather was returning AEC2 instead. This
> CL restores the original behavior.
> 
> However, this bug made so that crrev.com/c/1402803 did not fail any test
> when the default value was switched. Hence, this CL also addresses this by
> fixing the default value assumed in the select-settings unittests.
> 
> BUG=926158
> 
> Change-Id: If0ff62607ecb227c773af94bdf2ca9410c1e59fc
> Reviewed-on: https://chromium-review.googlesource.com/c/1442912
> Commit-Queue: Armando Miraglia <armax@chromium.org>
> Reviewed-by: Guido Urdaneta <guidou@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#627156}

Bug: 926158
Change-Id: I9e11e6b21fac23c8909a6b629c368392946fa4f8
Reviewed-on: https://chromium-review.googlesource.com/c/1445878
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#627508}(cherry picked from commit 357f9ddb751e098a1aab666ae5ae08804772d3ff)
Reviewed-on: https://chromium-review.googlesource.com/c/1449734
Cr-Commit-Position: refs/branch-heads/3683@{#135}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
4 files changed