Correctly handle system metrics with e.g. --force-device-scale-factor.

Because GetSystemMetrics() always reflects the true current system DPI, it's not
appropriate to scale its result by GetDPIScale(); this results in the wrong
value when GetDPIScale() is forced to return a scale factor different than the
native one.  (It's not 100% clear to me whether the "clamp <=1.25 to 1" code
should still be applied in this case, but I think not.)

However, simply changing GetSystemMetricsInDIP() to use
GetUnforcedDeviceScaleFactor() is insufficient, because that function will
preferentially return |g_device_scale_factor|, which was being initialized to
the result of GetDPIScale() in two places.

This doesn't actually make any sense when you think through it.
* When there was a forced device scale factor, this meant that
  GetUnforcedDeviceScaleFactor() was returning this forced scale.  This makes no
  difference to anything except the updated version of GetSystemMetricsInDIP(),
  which would be unable to access the non-forced scale factor.
* When there wasn't a forced device scale factor, and the unforced DPI was 1.0
  or >1.25, this simply meant that GetUnforcedDeviceScaleFactor() would return,
  as a cached value, the value it would have returned anyway.  The extra caching
  doesn't buy anything meaningful performance-wise since GetDPI() already caches
  internally, so this would have no discernable effect.
* Finally, when there was no forced device scale factor and the DPI was between
  1 and 1.25, this would mean GetUnforcedDeviceScaleFactor() would display the
  same clamping behavior GetDPIScale() would.  Again, this makes no difference
  to anything except the updated version of GetSystemMetricsInDIP().

So, without my change in GetSystemMetricsInDIP(), the two calls to
InitDeviceScaleFactor() I've removed will have no behavioral effect; with that
change, removing them is required in order to get the desired behavior, namely,
what the name "GetUnforcedDeviceScaleFactor()" actually suggests.

BUG=none
TEST=On Windows, running with --force-device-scale-factor=2 should result in a height above the main toolbar that's exactly double the normal height, instead of being shorter.

Review URL: https://codereview.chromium.org/1520283005

Cr-Commit-Position: refs/heads/master@{#365784}

1 files changed, 1 insertions, 1 deletions

diff --git a/ui/aura/demo/demo_main.cc b/ui/aura/demo/demo_main.cc
index 689f87d..b76dde2 100644
--- a/ui/aura/demo/demo_main.cc
+++ b/ui/aura/demo/demo_main.cc
@@ -129,7 +129,7 @@ int DemoMain() {
   gfx::GLSurface::InitializeOneOff();
 
 #if defined(OS_WIN)
-  gfx::InitDeviceScaleFactor(1.0f);
+  gfx::SetDefaultDeviceScaleFactor(1.0f);
 #endif
 
   // The ContextFactory must exist before any Compositors are created.