[Impeller] Turned on wide gamut support by default. (flutter/engine#39801)

After having run a memory audit on Flutter Gallery I believe we can turn
on wide gamut support by default for impeller. Here are the
ramifications of this change:
1) If an opaque surface is used and no transparent wide gamut images are
used, there should be no change in performance or memory usage.
1) If a transparent surface is used; the surface will take up 2x memory
(ex 4MB on iPhone 7) and will have a runtime performance cost.
1) If wide gamut **transparent** images are used, they will take up 2x
memory. Opaque wide-gamut images will take up the same amount of memory.

## Flutter gallery test results

### Test 1

Steps:
* Tap "Material"
* Scroll down and back to "Cards"
* Tap "Cards"
* Scroll to bottom of "Cards" example

Notice that the card images are wide gamut, AdobeRGB.

**Memory increase:** 10.401 MB (+8.7%)

### Test 2

Steps:
* Just launch the gallery

Notice that there shouldn't be any wide gamut images.

**Memory Increase:** 1.22 MB (+1%, this is probably a fluke of
measurement)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[Flutter Style Guide]:
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[testing the engine]:
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engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm

@@ -200,10 +200,9 @@ flutter::Settings FLTDefaultSettingsForBundle(NSBundle* bundle, NSProcessInfo* p
   settings.may_insecurely_connect_to_all_domains = true;
   settings.domain_network_policy = "";
 
-  // Whether to enable Impeller.
+  // Whether to enable wide gamut colors.
   NSNumber* nsEnableWideGamut = [mainBundle objectForInfoDictionaryKey:@"FLTEnableWideGamut"];
-  // TODO(gaaclarke): Make this value `on` by default (pending memory audit).
-  BOOL enableWideGamut = nsEnableWideGamut ? nsEnableWideGamut.boolValue : NO;
+  BOOL enableWideGamut = nsEnableWideGamut ? nsEnableWideGamut.boolValue : YES;
   settings.enable_wide_gamut = enableWideGamut;
 
   // TODO(dnfield): We should reverse the order for all these settings so that command line options