Revert "Increase the UI paint timeout to 5 seconds."

This reverts commit 51181f47.

Reason for revert:
Speculatively revert for resolving the bot failure on the CI: 
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.12%20Tests/14387
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.11%20Tests/28041

Original change's description:
> Increase the UI paint timeout to 5 seconds.
> 
> This fixes the first browser window appearing blank for a moment. In
> general, it's better to block than to show the UI in an inconsistent
> state. Web content should *not* wait this long.
> 
> Bug: 849762
> Change-Id: I2b73e52a0425ee6fa583fcddc0300dc8a2d3ad73
> Reviewed-on: https://chromium-review.googlesource.com/1135119
> Commit-Queue: Sidney San Martín <sdy@chromium.org>
> Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
> Reviewed-by: ccameron <ccameron@chromium.org>
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#574731}

TBR=ellyjones@chromium.org,ccameron@chromium.org,sdy@chromium.org

Change-Id: I80bd11b7478bd1b40b11c9c71aa75de65a874da9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 849762
Reviewed-on: https://chromium-review.googlesource.com/1136131Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574823}

ui/views/cocoa/bridged_native_widget.mm

@@ -51,7 +51,7 @@ CGError CGSSetWindowBackgroundBlurRadius(CGSConnection connection,
 
 }
 namespace {
-constexpr auto kUIPaintTimeout = base::TimeDelta::FromSeconds(5);
+constexpr auto kUIPaintTimeout = base::TimeDelta::FromMilliseconds(250);
 }  // namespace
 
 // The NSView that hosts the composited CALayer drawing the UI. It fills the