Make sure line widths in the range of 0px and 1px are rounded up to 1px for visibility.

third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h

Index: third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h b/third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h
index e404ef65c614cd2cb476a38b9f45699938cc5675..3109d5535f17b8da729146de31f5e514e981d351 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h
+++ b/third_party/WebKit/Source/core/css/resolver/StyleBuilderConverter.h
@@ -135,15 +135,10 @@ T StyleBuilderConverter::convertLineWidth(StyleResolverState& state, const CSSVa
     if (valueID == CSSValueThick)
         return 5;
     if (valueID == CSSValueInvalid) {
-        // Any original result that was >= 1 should not be allowed to fall below 1.
-        // This keeps border lines from vanishing.
-        T result = primitiveValue.computeLength<T>(state.cssToLengthConversionData());
-        if (state.style()->effectiveZoom() < 1.0f && result < 1.0) {
-            T originalLength = primitiveValue.computeLength<T>(state.cssToLengthConversionData().copyWithAdjustedZoom(1.0));
-            if (originalLength >= 1.0)
-                return 1.0;
-        }
-        return result;
+        double result = primitiveValue.computeLength<double>(state.cssToLengthConversionData());

[eae, 2015/11/02 03:02:02]

I'd argue that a border that is specified to be 0.25 should round to zero rather
than one. Today that doesn't really matter but with the move to use full page
zoom for high-dpi 0.5 would map to 1 device pixel on high DPI. At the very least
I'd like us to clamp it to device pixels or zoom adjusted CSS pixels instead of
raw CSS pixels.

[eae, 2015/11/02 03:14:46]

It's probably more important to match the other browsers here than to match our
own legacy behavior. Let's give this a try. Please add a comment about hi-dpi
support though and reference bug 485650.

+        if (result > 0.0 && result < 1.0)
+            return 1.0;
+        return clampTo<T>(roundForImpreciseConversion<T>(result), defaultMinimumForClamp<T>(), defaultMaximumForClamp<T>());
     }
     ASSERT_NOT_REACHED();
     return 0;