Revert "Clipboard: Remove unnecessary ClipboardBuffer argument"

This reverts commit 9f2e13b917fe46c87cb3528e7658be5ad533e110.

Reason for revert: Regression https://crbug.com/959765

Removal of SystemClipboard::ReadPlainText logic checking and passing
through |buffer_| resulted in a regression where attempting to middle
click paste an X11 selection clipboard would incorrectly paste the
copy/paste clipboard. The change was incorrect because blink::ClipboardCommands::ExecutePasteGlobalSelection() did indeed change the active clipboard buffer.

Original change's description:
> Clipboard: Remove unnecessary ClipboardBuffer argument
>
> of mojom::ClipboardBuffer::kStandard. While it could be useful
> to allow this argument to have other values, perhaps to use
> for ClipboardBuffer::kSelection in X11, this is currently not
> used and always has the same value, so let's remove the unnecessary
> checks and passing around of this buffer value.
>
> SystemClipboard: :Read{Image,PlainText}() have an argument,
> mojom: :ClipboardBuffer buffer, which always has the same value
> Change-Id: Ide1d9ecb5b8f2869b14232dc2acbfe4c681419e5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576184
> Commit-Queue: Victor Costan <pwnall@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#653521}

TBR=pwnall@chromium.org,huangdarwin@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I5870d6e8d81e5e412848e6d39be1015980b94ab3

Bug: 959765
Change-Id: I5870d6e8d81e5e412848e6d39be1015980b94ab3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606425
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658785}
5 files changed