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1. Calculate the hash in only two places: `content_aware_hash.{ps1|sh}`
2. Call this from the workflow
3. Eventually call this from `update_engine_version.{ps1|sh}`

 The files of import:
* `DEPS`: tracks third party dependencies related to building the engine
* `engine`: all the code in the engine folder
* `bin/internal/content_aware_hash.ps1`: script for calculating the hash
on windows
* `bin/internal/content_aware_hash.sh`: script for calculating the hash
on mac/linux
* `.github/workflows/content-aware-hash.yml`: github action for CI/CD
hashing

Tested on windows and mac:

```shell
PS C:\src\flutter> C:\src\flutter\bin\internal\content_aware_hash.ps1
c24231e276e0719738e175e0622e040ad21a7012
```

```shell
❯ ~/src/flutter/bin/internal/content_aware_hash.sh
c24231e276e0719738e175e0622e040ad21a7012
```

…)" (flutter#166864)

<!-- start_original_pr_link -->
Reverts: flutter#166717
<!-- end_original_pr_link -->
<!-- start_initiating_author -->
Initiated by: matanlurey
<!-- end_initiating_author -->
<!-- start_revert_reason -->
Reason for reverting: Broke tree on
[postsubmit](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8718108088961866849/+/u/test:_run_suite_safari-dart2js-canvaskit-engine/stdout)
<!-- end_revert_reason -->
<!-- start_original_pr_author -->
Original PR Author: jtmcdole
<!-- end_original_pr_author -->

<!-- start_reviewers -->
Reviewed By: {matanlurey}
<!-- end_reviewers -->

<!-- start_revert_body -->
This change reverts the following previous change:
1. Calculate the hash in only two places: `content_aware_hash.{ps1|sh}`
2. Call this from the workflow
3. Eventually call this from `update_engine_version.{ps1|sh}`

 The files of import:
* `DEPS`: tracks third party dependencies related to building the engine
* `engine`: all the code in the engine folder
* `bin/internal/content_aware_hash.ps1`: script for calculating the hash
on windows
* `bin/internal/content_aware_hash.sh`: script for calculating the hash
on mac/linux
* `.github/workflows/content-aware-hash.yml`: github action for CI/CD
hashing

Tested on windows and mac:

```shell
PS C:\src\flutter> C:\src\flutter\bin\internal\content_aware_hash.ps1
c24231e276e0719738e175e0622e040ad21a7012
```

```shell
❯ ~/src/flutter/bin/internal/content_aware_hash.sh
c24231e276e0719738e175e0622e040ad21a7012
```
<!-- end_revert_body -->

Co-authored-by: auto-submit[bot] <[email protected]>

Rolls forward flutter#166717.

Does not copy the `README` or `DEPS` files, and instead uses synthetic
scratch files.

These files can change, so we can't possibly know how to hash them
consistently.

…er#166717)

…er#166717)

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