Avoid BOT/EOT setting in HarfBuzz.
Avoid BOT/EOT setting in HarfBuzz.

Unlike many users of HarfBuzz, the Skia shaper sets HB_BUFFER_FLAG_BOT
and HB_BUFFER_FLAG_EOT flags to inform HarfBuzz that the full context
contains the beginning and end of the paragraph so that it can treat the
beginning and end of the context specially. In reality, the EOT flag
currently does nothing and the BOT flag only has the effect of adding a
dotted circle (if the font provides it) if the first codepoint in the text
and context is a unicode mark. This behavior is generally unwanted, so
just remove it with a note to revisit this decision should HarfBuzz ever
change the effect of these flags.

Bug: skia:9618
Change-Id: I6cdf86ff3499e1321b1212d63192a4a9c5847e39
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264686
Auto-Submit: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Julia Lavrova <66a91ddff0a66e6df510780220f7ed6687af5ca5@google.com>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,google/skia