Reland #19396 with a fix for improper scale that was affecting internal tests

Tested: Ran all unittests, ran internal tests, and ran workstation on Fuchsia
BUG: 53062, 53063

* Handle glfwGetPrimaryMonitor returning nullptr

[glfwGetPrimaryMonitor](https://www.glfw.org/docs/3.3/group__monitor.html#ga721867d84c6d18d6790d64d2847ca0b1) may return NULL in non-error cases when no monitor is found. This started to happen recently with an update to the linux distribution, see internal b/162545223.

Handling this case avoids a crash and instead assumes a pixel density.

controllers instead of views.

* Make sure ios tests fail if they fail

* explain

hasStrings message for checking for pasteable clipboard contents without actually reading them, for iOS14 clipboard alerts.

A recent refactoring broke the USE_LEGACY_ENCODABLE_VALUE codepath in
standard_codec.cc, which went unnoticed since it wasn't being compiled.
This fixes the breakage, and also adds a temporary minimal unit test
target that ensures that all the USE_LEGACY_ENCODABLE_VALUE paths are
being compiled.

Related issue: https://github.com/flutter/flutter/issues/56719

Remove the dummy rasterizer delegate now that flutter_runner is in tree, and cleanup ctor params (#20486)

* add enabled flag

* update doc

* update doc

* Clean-up CanvasKit canvas sizing.

* cache size correctly