[Merge 136][Read aloud] Fix skipping some text in pdfs

The issue was that GetNextSentence was returning the same AXPosition so
we were skipping to the next paragraph which skipped text. I added a
check for next line in pdfs since pdfs are sometimes laid out that way.

(cherry picked from commit 0b84e889146f3032b5e29dbff22814e72cd37efd)

Fixed: 410065627
Change-Id: I69ad804bab95e5377050f5716b5f12f46a39706b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6435035
Reviewed-by: Lauren Winston <lwinston@google.com>
Auto-Submit: Kristi Saney <kristislee@google.com>
Commit-Queue: Kristi Saney <kristislee@google.com>
Commit-Queue: Lauren Winston <lwinston@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1442986}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6448659
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Jocelyn Tran <jocelyntran@google.com>
Commit-Queue: Shari Trewin <trewin@google.com>
Cr-Commit-Position: refs/branch-heads/7103@{#854}
Cr-Branched-From: e09430c64983fc906f37a9f7e6806275c9b67b86-refs/heads/main@{#1440670}