Reland "account for inline placeholders in longest line calculation (#20370)" reverted in (#20669)

[LongCatIsLooong]

Related: flutter/flutter#64256

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[LongCatIsLooong]

• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12212
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12216
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12219
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12221
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12222
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12223
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12224
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12227
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12233
• https://ci.chromium.org/p/flutter/builders/try/Linux%20Host%20Engine/12235