[tools] Fix progress bar going backwards

Maths was wonky, causing the blocks to go back one on a whole integer.

Change-Id: I1f7d49c1d1b12050dd48b2090dbeff9d4a1c6c38
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/133600
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

tools/src/progressbar/progressbar.go

@@ -187,7 +187,7 @@ func (s Status) Draw(out io.Writer, width int, ansiColors bool, animFrame int) {
 				fmt.Fprint(buf, strings.Repeat(string("▉"), numBlocks-1))
 			}
 			if numBlocks > 0 {
-				frac := fNumBlocks - math.Floor(fNumBlocks)
+				frac := fNumBlocks - float64(numBlocks-1)
 				symbol := blockSymbols[int(math.Round(frac*float64(len(blockSymbols)-1)))]
 				fmt.Fprint(buf, string(symbol))
 			}