Do not add semicolon to the last declaration if there was none
Do not add semicolon to the last declaration if there was none

Previously when we update style text, when there is no semicolon at the
end of the *last* CSS declaration in a declaration block, we would add
a semicolon. So the following modification happens:
```
div { color: red }
```
becomes
```
div { color: red; }
```
This would bring unnecessary changes to authored CSS styles, and the
change would also be registered in the Changes tab. This CL removes
this automatic appending (only to the *last* declaration, otherwise
it's invalid CSS) under the preciseChanges experiment.

Bug: chromium:1238818, chromium:1268754
Change-Id: Ib9567dc9c881bc5b42f5df0f984aaf99c5981958
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3306493
Commit-Queue: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>
Auto-Submit: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>
Reviewed-by: Johan Bay <c4a91c97e1f8a218202ca733f9e9f5122f475e0f@chromium.org>

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