[css-typed-om] Simplify CSSUnitValue conversion.

Currently, when we convert a CSSUnitValue to a different unit, we
essentially consult a NxN table to figure out the scale factor, where
N is the number of units.

An easier way is to first convert to the canonical unit, then convert
from the canonical unit to the target unit. This only requires a lookup
table of size N instead of NxN, greatly simplifying the code.

Bug: 776173
Change-Id: Icb28386938f9cb7de4d917c72426154b8f45d6e6
Reviewed-on: https://chromium-review.googlesource.com/798930Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520848}

third_party/WebKit/LayoutTests/typedcssom/stylevalue-subclasses/numeric-objects/to.html

@@ -24,8 +24,11 @@ test(() => {
 }, 'Converting a CSSUnitValue to an incompatible unit throws TypeError');
 
 test(() => {
-  assert_style_value_equals(CSS.number(1).to('number'), CSS.number(1));
-  assert_style_value_equals(CSS.px(1).to('px'), CSS.px(1));
+  for (const unit of gValidUnits) {
+    // FIXME(778495): Remove this check onec all the units are supported.
+    if (CSS[unit])
+      assert_style_value_equals(CSS[unit](1).to(unit), CSS[unit](1));
+  }
 }, 'Converting a CSSUnitValue to its own unit returns itself');
 
 // TODO(776173): cssUnitValue_toMethod.html has more comprehensive tests of converting