win: Rename clang_x64 to win_clang_x64 in build/toolchain/win, step 5/5.

In cross builds of chrome/win, the host and target toolchains currently
have the same name.  To fix this, rename clang_x64 to win_clang_x64.
Because the toolchain name is also referenced in libyuv, this requires
a five-sided change:

1. Introduce variable containing the toolchain name in src.git
2. Change libyuv to refer to the variable
3. Rename toolchain in src.git (including in the newly introduced var)
4. Let libyuv refer to the new name directly
5* Remove variable again

(See also https://codereview.chromium.org/2463143002)

Bug: 748501
Change-Id: I8fe3557fb6a200116ba4c4d6dc3338f895fd3757
Reviewed-on: https://chromium-review.googlesource.com/585592
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#489426}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9f072955094ffe6fbab3e864286ab6b1dcdf8f8c
1 file changed