[Renderer] Simplify blurredVision deficiency filter

This patch removes a redundant attribute/value pair from the
internal blurredVision filter definition.

`in="SourceGraphic"` is the implicit default per [1] and [2]:

> If no value is provided and this is the first filter primitive,
> then this filter primitive will use SourceGraphic as its input.

[1]: https://www.w3.org/TR/SVG11/filters.html#CommonAttributes
[2]: https://drafts.fxtf.org/filter-effects-1/#CommonAttributes

Bug: chromium:1003700
Change-Id: Idc2aaaf435e5ef57e00cfc0341d53214812001c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094667Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749167}

third_party/blink/renderer/core/css/vision_deficiency.cc

@@ -41,8 +41,7 @@ AtomicString CreateVisionDeficiencyFilterUrl(
           "0.000 0.000 0.000 1.000 0.000 "
           "\"/>");
     case VisionDeficiency::kBlurredVision:
-      return CreateFilterDataUrl(
-          "<feGaussianBlur in=\"SourceGraphic\" stdDeviation=\"2\"/>");
+      return CreateFilterDataUrl("<feGaussianBlur stdDeviation=\"2\"/>");
     case VisionDeficiency::kDeuteranomaly:
       return CreateFilterDataUrl(
           "<feColorMatrix values=\""