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This allows overriding the user agent if an override is set.

NOTE: This does require the caller to override the user agent in order
for it to happen. If the caller doesn't override, the default user agent
is used.

Bug: b:175857659
Test: cast_shell_browsertests
Change-Id: I713f01076baabe108b17e1d4934f7dc4259b7139
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633705Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844585}