[WebGPU] Fix ImageHelper::flushSingleLevelUpdates This change modifies ImageHelper:flushSingleLevelUpdates to only send a flush command to the context once, instead of in each iteration of the loop if there are multiple texture uploads. This also means that the last texture upload is guaranteed to be flushed, as otherwise that texture could be staged in the queue, but a different command could redefine texture levels which would delete the underlying texture handle but not the command queued for that texture. Bug: angleproject:438268609 Change-Id: I2d49b15fc921829175e99bdb550dddbd81179061 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6885223 Commit-Queue: Matthew Denton <mpdenton@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>