[console] make context selector subtitle contrast WCAG compliant
[console] make context selector subtitle contrast WCAG compliant

The console context selector subtitle displays useful product information
to help differentiate between contexts. This feature was added in response
to https://crbug.com/719058.

In the original implementation a text foreground color value of #999 was
used (light theme) which does not meet WCAG 1.4.3 compliance:
https://www.w3.org/WAI/WCAG21/quickref/#contrast-minimum

Changing the CSS color value to use `--selection-inactive-fg-color` sets
the color value to #5a5a5a in light theme and #cdcdcd in dark theme, which
meets contrast minimum.

Before (Light Theme) Colors: https://webaim.org/resources/contrastchecker/?fcolor=999999&bcolor=FFFFFF
Before (Dark Theme) Colors: https://webaim.org/resources/contrastchecker/?fcolor=666666&bcolor=242424

After (Light Theme) Colors: https://webaim.org/resources/contrastchecker/?fcolor=5A5A5A&bcolor=FFFFFF
After (Dark Theme) Colors: https://webaim.org/resources/contrastchecker/?fcolor=CDCDCD&bcolor=242424

Additionally using the CSS variable makes the tool more theme-able,
`--selection-inactive-fg-color` was chosen because the item colored
is an inactive selection and aligns with usage in the tree outline control [1].

It took me a minute to get used to but I now prefer the new colors because
of how easy it is to see all the information, especially in dark theme.

[1]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/devtools-frontend/src/front_end/ui/treeoutline.css?originalUrl=https:%2F%2Fcs.chromium.org%2F

Screenshot: https://imgur.com/sZ4iAvX
Change-Id: Ic97fdef1ad2ed6989dcc9e6d7263b28babc077a9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2202665
Commit-Queue: John Emau <c153e6e600be23fb28cec0b08551440e4bf7575d@microsoft.com>
Reviewed-by: Kalon Hinds <0953a07de5dd99912b5643571c8ed9864c017a6d@microsoft.com>
Reviewed-by: Tony Ross <d5807c9a1621d46cc76cd6770054768aff0ae774@microsoft.com>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

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