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Add new toolchains for using 14.0 deployment target. This will be
used by Widget extensions (which needs to have that deployment
target).

Add new gn variables to detect if a secondary toolchain exists for
"fat" build or not. This was previously done by checking whether
the current_toolchain is the default_toolchain but this is no longer
valid with toolchains used for other purposes.

Bug: 1138714
Change-Id: I7705dafd2d6273a0cc77d78e5630d5c8651ea3d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504229
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mike Pinkerton <pinkerton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822121}