Fix slow initial bootstrap on Windows.

Invoke-WebRequest shows a Powershell progress bar which absurdly
slow downs the download. See [1] for related discussion.

Test on a fresh checkout:
Measure-Command { cmd.exe /c update_depot_tools.bat | Out-Default }

Before: TotalSeconds : 102.4310994
After:  TotalSeconds : 36.3454104

[1] https://stackoverflow.com/questions/28682642/powershell-why-is-using-invoke-webrequest-much-slower-than-a-browser-download

Change-Id: I1c6e0200f2b140e9b4c23bd80e8565ff73ee7584
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4532007
Auto-Submit: Aleksey Khoroshilov <akhoroshilov@brave.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

.cipd_impl.ps1

@@ -106,6 +106,7 @@ $TmpPath = $CipdBinary + ".tmp." + $PID
 try {
   Write-Output "Downloading CIPD client for $Platform from $URL..."
   Retry-Command {
+    $ProgressPreference = "SilentlyContinue"
     Invoke-WebRequest -UserAgent $UserAgent -Uri $URL -OutFile $TmpPath
   }