[iOS] Enable the printing feature for web tests

iOS Blink port doesn't enable the printing feature yet because of
unimplementations of PrintingContext and PrintSettingsInitializer
classes used by //chrome. Until enabling the feature, this CL
enables the printing feature to run the web tests related to
printing.

Additionally, this CL renames 'enable_printing_unittests' to
'enable_printing_tests' because the flag is enabled for
unit tests and web tests.

Bug: 1421239, 1432082
Change-Id: I74ef6b4040ab99d875edf95f19dd474e374bc450
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4994944
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Kevin Lubick <kjlubick@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1220935}